1. I personally don't run any AV crap, just surf safely. Well, let me rephrase. I do have it installed, just not running. I use it to scan specific files.
2. In my experience/opinion, regardless a PC will simple need the OS wiped and reinstalled after a year, 2...3...4 or whatever. That's just the way windows rolls. Various things cause this and I don't know if that's the case with MAC OS or Linux OSs.
3. Possible you have hardware going bad.
4. One thing that may help.
a. Delete all unused/needed data from your drive.
b. Backup the rest of your data (should always be done anyway)
c. Open up a cmd prompt and type "defrag -v c:" without the quotes and let it go to town for a few hours. Sometimes that helps.
My '03 Sold.