Thanks for the helpful advice! But, now my problem has been solved. You will probably wondering how I did it? So, here it goes! I formatted my hard drive and installed a fresh copy of Windows 7 (32-bit). And up till now its working fine. But anyway thanks for the helpful advices "seth500"!
There's one more thing I would like to ask you that, is it recommended to install sevice pack 1 for Windows 7?