1) Apart from Btcoin-Qt which is the official client of BitCoin, there are several other clients which include CGMiner, Phoenix, BFGMiner, BTCMiner, DiabloMiner, etc? I am not sure about this, I have downloaded those programmes and it looks like I can run only Btcoin-Qt successfully.
Bitcoin-qt is the official Bitcoin GUI client, which you use for sending and receiving Bitcoins. The mining option has been removed from the client long time ago, because without special hardware you will not be able to generate any significant amount of Bitcoins any more. The other programs are mining software which connect to the Bitcoin client (run in server mode) or a mining pool.
2) I tried generating myself some bitcoins through Btcoin-Qt, and it takes about 4hours+ to complete 180,000+ blocks. Now it has completed, and I didn't see any BTC in my Wallet and its not generating any new blocks. Do I have to wait few hours for the balance to appear in my wallet and it will start generating new blocks?
You were looking at the block chain downloading. After it had completed you didn't generate any new Bitcoins and nobody has sent you any yet, so your balance is zero. Sorry
3) How do I find out what is my bitcoin wallet address?
Click on Receive Coins and use one of the addresses or create new addresses (your wallet can have many different addresses which can receive your Bitcoins).
4) I see many people talking about mhash/sec, but I don't see any option of it in the Btcoin-Qt client. How can I check for my mhash?
There is no mining option in the GUI client anymore. See 1)
5) How many mhash/s is needed to generate 1 BTC over a 24 hours period?
An the moment roughly 2 gigahash/s (2000 mhash/s) would generate 1 BTC in 24 hours. This increases as more people start mining.
6) I see there is a service offer here, that if you deposit bitcoins with them, they offer you a 6.9% interest every week. Would that be an better option as an investor, to just purchase bitcoin from an exchanger and deposit it with them? This pretty much take away the headache of cooling, hardware, electricity and stuff..
Depositing with a third party always has risks. See the recent Bitcoinica example. If you want to invest, the easiest way is to buy your Bitcoins (anywhere), store them in your own secure wallet, invest a lot of time on learning how to secure it (backup and encryption) and wait a couple of years. Mining is more profitable but it requires an investment in hardware and (a lot of) time keeping it running and tuning.
Sorry all, have been too confused after reading up so much stuff of bitcoins and still not understand a single thing.. appreciate if anyone could help! thank you
No problem, hope that helps you.