Problems with Bitcoin-qt wallet

[pkk2000]

I am looking for a guide to use this bitcoin wallet http://bitcoin.org/en/download .
I never made any password or username, I just ran the program and it worked without no login.
I rechieved money on it, but I wonder why it does not ask me for any username or pass when i start the program, even after i reboot my computer.
It automaticly signs me in to my account when started. Will that say that I can only get to my wallet by using this program on this excact computer?

I also chose to encrypt my wallet, and now I cant find any way to decrypt it again.
As mentioned above, a guide to this kind of wallet is much appriciated.

[DannyHamilton]

Will that say that I can only get to my wallet by using this program on this excact computer?

Yes.

I also chose to encrypt my wallet, and now I cant find any way to decrypt it again.

It is not currently possible to remove encryption from an encrypted Bitcoin-Qt wallet.

Now that you've encrypted the wallet, it will be impossible to send bitcoins out of the wallet without providing the encryption password.  So, while anyone with access to the computer may be able to start up the wallet and see how much is in there, they won't be able to steal/spend any of those bitcoins unless they know your password.

Do not forget your password.  There is no way to "reset" the password.  There is nobody who can "retrieve" your password for you.  If you forget your password, your bitcoins will remain unspendable unless/until you can remember it.

Note:  As long as you know the current password, it is possible to change the password to something new.