If you are on Vista or Win7, paste the line
"\Program Files (x86)\Bitcoin\bitcoin-qt.exe" -rescan
into the start menu search bar. (remove the " (x86)" if you are using 32 bit Windows). It will take several minutes for Bitcoin to start up because it will scan for missing transactions before presenting the user interface.

The latest version of Bitcoin has changed the way that previously connected clients are cached. In older versions, in the Bitcoin data directory, the file addr.dat contained a list of known peers so that you could quickly restart. The newest version of Bitcoin creates a file peers.dat that contains the same information. Perhaps one of these files is corrupted and the peer-to-peer component can't start correctly - deleting them both and restarting may be the solution.

Antivirus (Microsoft security essentials) may have it's own crap firewall that doesn't let unapproved software make connections. Discover if any AV package has a firewall component and disable it.

Did you put port forwarding rules into your router? It may be that your desktop PC was assigned a different IP address than was previously valid for the rules - remove any port forwarding and be happy with 8 connections.

Finally, if there is an actual program fault when launching Bitcoin, it may be recorded near the end of the debug.log file or db.log file.

1. Ensure that really is your address, that it is listed in "Receive coins",

2. Close Bitcoin and the taskbar icon, and relaunch it with the command-line option bitcoin-qt -rescan, to search for payments to you,

3. Look under "Transactions" to discover payments in and out of your wallet to that or other addresses.

Relevant thread:
https://bitcointalk.org/index.php?topic=51456.0

You can explore to the Bitcoin data directory by simply typing %appdata%/bitcoin into the Windows 7 start menu search box.

In that directory you will see the file "wallet.dat" (or perhaps just "wallet" with the file type "DAT File" if you have file extensions hidden, which is the default on Windows). This is the file that contains your personal data that allows you to spend bitcoins. This file should be copied to a USB stick or other location so you will always have a backup copy in case your hard drive dies, your computer gets stolen, etc.

In the data directory, you will also see the files that contain the blockchain (blkindex.dat, blk0001.dat, and possibly blk0002.dat if you have gotten blocks past 190,000). These do not contain any personal information, they are a record of all transactions that have every happened on the Bitcoin network.

Often there will be hangs in downloading the blockchain from the database files getting corrupted while being downloaded. This could be potentially caused by corrupted information from other clients, software crashes, or disk problems. Upgrading to a different version of Bitcoin may also cause this symptom depending on the blockchain state at the time of the upgrade.

The solution is to completely close Bitcoin (including the tray icon), remove the old blockchain files, install the latest stable version of Bitcoin, and run Bitcoin and let it download the complete blockchain again. If you want to save many hours of downloading, you can download and extract blockchain files such as in the linked thread above before you run Bitcoin, and then it will only need to download a month of data instead of three years of data.

The starting point of a phrase search is random, it gets a private key from the openssl rand ecdsa library and starts searching from there, simply incrementing the key. After searching many addresses for a phrase match, it will again get another random private key. The keys searched and returned with a found vanity phrase in the corresponding Bitcoin address can be from anywhere in the range of valid keys, but certainly it cannot "search the entire keyspace", as that would take somewhere just north of the age of the universe.

I understand your point and agree fully!

You've never been hacked or scammed out of bitcoins or lost (or discarded) them or a wallet. There are about seven billion people in this club, so say hi if you actually have your own bitcoins in your own damn backed-up wallet and realize that sending money to anonymous people on the internet is a bad idea.

<- Member #1

Using an online wallet is trading all security for moderate convenience. Would you mail USD to anonymous entities you found on the internet for safekeeping? See mybitcoin (or a dozen other people/sites that have absconded with Bitcoins or lost them).

A few wallet file backups should be all you need to ensure that you don't lose your money. If you want to have more security against hackers and trojans, you can create an offline address or wallet backup that is only used for savings. This can be better than cash, because cash can't be stored in several locations to protect against theft, fire, etc. You don't need to have a client running for people to send you money or see the balance of an address if you plan to be away from your computer or the net.

You only need to download the blockchain once. It contains old payments from 2009 to now. 50% is actually about 10%, because Bitcoin has become more popular recently, and there are many recent transactions to download. After your first blockchain download finishes, only new information has to download each time you start the client.

Bitcoin, or "Bitcoin wallet", is the name of the client software you install. After you run the software, it will generate a Bitcoin address for you. You can give this address to other people, and they will be able to send you Bitcoins. Under "receive coins" in the Bitcoin software, you can generate more new addresses, so that you can give different address to different people. This is useful if you want easily determine who sent you money.

Once you run the software, it must download all transactions that have ever taken place, so that Bitcoin knows how much money every user of Bitcoin has. This can take many hours, or even a day or two on a bad internet connection. As a new user, you will only be able to see money that has been sent to you after all transaction blocks have been downloaded.

After you have received money, you can then make a payment to another user by sending coins to one of their Bitcoin addresses.

It is important that you make a backup of the wallet file, as only the computer you install Bitcoin on has the information you need to send coins - if your hard drive crashes, you will lose all your money, because you no longer will be able to send it to anyone. Please look at the wiki to learn how to make a backup of your wallet.dat file, which contains all your addresses.

How do you get bitcoins? You trade something for them, either money, services, or merchandise. There are currency exchanges where you can simply buy bitcoins on the internet. There also may be users near you that will sell Bitcoins in person, or may pay you Bitcoins for something. Sometimes there are people here that will even send you small amounts for free so you can see how Bitcoin works, simply give them your Bitcoin address.

You should become a super-expert in all aspects of Bitcoin before creating any website or internet service that uses it.

You must understand that Bitcoin is not a company or an organization. It is open source peer to peer software. This forum is made of Bitcoin users of various knowledge levels. Bitcoins are transferred peer-to-peer, there is no central authority, and nobody can interfere with or undo Bitcoins that you have sent. MtGox is a company, but they have no way to get back Bitcoins after they were sent either.

In your Bitcoin software client, is the status in the lower right "up to date" (a green checkmark)? If the Bitcoin client still says that it is downloading blocks, than it may simply be that you have not downloaded information about the money transfer yet, and the address you used is actually yours.

Otherwise, there is no alternative, except to try to figure out who the actual owner of that address is (perhaps someone you previously sent money to in your address book), and contact them and explain that you accidentally sent them money - can they please send their new mystery money to you (and give them a real address from your "receive coins" address list).

There is no registry or database of who owns what address. Googling will sometimes reveal an address's owner, if, for example, it is a donation address that is posted on the web.

Bitcoin has two lists of addresses: "Receive Coins", which is where you create your own addresses for people to send money to you, and "Address Book", where you can store addresses that belong to other people for future sending. Both of these screens give you the ability to label addresses so you can notate who sent you coins, or remember frequently used addresses that you send money to.

If you used an address from "address book" as the address to withdraw MtGox Bitcoins, you have essentially sent money to someone else. You should look at your own past transactions to locate who the owner of this address is, since the address is likely in your address book because you previously sent Bitcoins to the address's owner.

More price stability will come when 1 BTC will always buy you a pair of socks...

Selling or buying items priced in Bitcoins can also be beneficial whether you are optimistic or pessimistic about future values. If you wish to obtain Bitcoins because you think they will hold their value well, then selling merchandise is a great way to do so. As a long term investor, you have a price you are willing to pay for Bitcoins too, and selling one widget = one Bitcoin is better for you then selling for the current price on a volatile exchange. Likewise, if you are pessimistic about Bitcoin, you may find it better to buy the merchandise you want than to go through the time consuming fee-laden process of converting to and withdrawing a government-controlled currency, or may be more likely to buy if an exchange says you are getting a good deal.

The public key is essentially the Bitcoin address.
The private key is the un-seen complement to the address that allows spending coins.

Vanitygen generates a random public-private pair, and then sees if it has the vanity phrase in it. If not, it discards the result and tries again. About a million times a second.

It is impossible to find the private key for a Bitcoin address if you don't have it. That would allow spending that address's funds.

For details specific to vanity key pooling, please see the linked thread. Also https://vanitypool.appspot.com/faq

Microsoft has a slightly different opinion:

or

Start with the command line option -maxconnections=X, where X is the largest number of other peers you want to maintain connections with. This won't prevent one of the clients wanting to get the whole blockchain from you, but it will lower the chance.

The Bitcoin client does not simply download blocks and store them. It adds information about blocks, transactions, and addresses to a database with an index, and it is the building of this index, along with verification of the cryptographic hashes and signatures of all transactions that have taken place, which heavily utilizes storage and CPU resources.

If you wish to re-download the blockchain for the sake of re-verification, you can create a 4GB RAM disk (if you have 6+ GB of RAM), and utilize this as your -datadir. This will remove the bottleneck of limited I/O operations per second from a spinning hard drive with moving heads. Then you will be limited by the CPU speed of the system running Bitcoin. Of course move the downloaded blockchain to a more permanent location before powering off your computer...

Later blocks have many more transactions, thus take longer to download, the top graph shows the current block number after the elapsed time downloading, the bottom graph shows the blocks per second at a particular block height (notice how abysmal it gets at the very end):

In bitcoin-qt, the menu option Settings->Encrypt Wallet

If you have sufficient connections to other nodes, then database corruption is likely. In the %appdata%\Bitcoin data folder, check the end of db.log and debug.log, which will have normal logging until near the end and then the error should be apparent.

The -rescan option only looks for missing payments to you, it does nothing to discover blockchain errors.

The command line option that may actual detect (repair?) errors is checkblocks. The options below give the most extensive check possible, which will take a long time, but slightly less than redownloading the blockchain:

>bitcoin-qt.exe -checkblocks=0 -checklevel=6

Pet hair dye...