What new programs have you possibly either downloaded and installed in the past week? Have you attempted to update any drivers (either video or sound card ones?) Think about this. (Also, anyone else been messing with your computer in the last little while other than you?)

If everything was working fine before, then suddenly you're getting blue screens of death, or the type of problems you've described, could be because of something that's overwritten a DLL file, or a bad driver.

You could also try going into System Tools, and seeing if you can restore your registry (registry must have become corrupted), or have Windows check for corrupted files and replace those as required. (You'll need your Windows CD for this.)

Sadly, if all else fails, try and back up what you can, and reforat the h.d. and reinstall clean, from scratch. Pain in the ass, but sometimes the only way to restore things to a clean bill of health.