Have you tried scanning for virus spyware, Scanned your Hard drive for bad sectors, run diagnostics on the Hard Drive by visiting the manufacture website, Made sure there is no dust build up in your computer. defrag your hard drive.
Failing this try to restore the computer back in time to when you knew it was working by using system Restore.
if still no joy download and run Memtest, it needs to be extracted to a clean floppy and booted, run the test for a few hours.
If this fails to work it maybe that you have a corrupted registery and will need to reinstall windows.