Vishnu, we definitely don't have much information to go on. However, it is not uncommon for an end user not to be able to provide much more information that what you have provided. Here is how I'd suggest you go about it.
1. Backup your data first. If you can ghost the entire drive, all the better.
2. Verify that all your critical data is backed up. (e-mail, address book, documents, etc)
3. Test your computer's hardware (hard drive, RAM, CPU, System Board, etc). There is no point in fighting with the Windows installation, if you problems are hardware related. You will simply just make things worse.
4. Remove any uneccary hardware (printer, MP3 player, etc) from the computer then turn it on
5. Attempt to boot Windows to a previous restore point
6. Attempt a repair installation of Windows
7. Verify the backup one more time, make sure you have all the necessary apps and drives, then wipe and reload
All in all, you have your work set out for you. If you are unsure of what you are doing, I suggest spending the money and paying a technician who has the tools to help you out. Just make sure they backup your data before they do anything...you don't want to need my
data recovery services if they mess up or your drive crahsed in the process.
If, at any point in time, you find that you are unable to get the data off the drive and it is important to you, it is a good time to stop and talk to a
data recovery professional . I can't speak for other companies, but mine will take the time to discuss your options on the phone and help you come up with a game plan. And, as with most companies, if you choose to send your hard drive in, the evaluation is free and there is a no charge if your data is not recoverable.
Best of luck to you and may you have a very happy New Year!
Luke
Lost Data? We get it back!