I was just running fine too.
If you don't hear any strange noises (like clicking, grinding, drive motor spinning up and down constantly), but hear normal sounds from your harddrive, you can try booting from a Windows install cd, selecting the recovery console, and then typing "chkdsk /r". This will check the current partition/driveletter for issues and (hopefully) fix them. Or at least enough to boot your computer, backup important data, and either reinstall or replace the drive.
If you are hearing clicking sounds, that usually means a mechanical issue with the harddrive. I've heard about people taking their harddrives out of the computer, placing them in a ziplock bag and putting them in a freezer for various lengths of time (avg I keep seeing is 30 minutes). Then quickly take drive out, put back into the computer and boot. Hopefully this will stop the drive clicking long enough to backup your data. If you hear grinding, most likely a head crash inside the drive and it's not worth fixing.
You can also try running the diagnostic cd that all of the major harddrive manufacturers have. Each one has their own diagnosics that you burn to a cd, boot from the cd, and then check your harddrive. It will run various tests on the drive. If it fails the tests, then most likely the drive is going bad.