When was the last time you did a fresh reinstall of your OS and programs on the machine?
I do a complete reinstall usualy every year when I feel like my PC would benefit, performance wise, with a clean slate. At most I've gone 2 years w/o doing it, I think one year I went through a new game every two months, so I reinstalled my OS three times. I still have a couple programs I install and run regularly/weekly to keep performance up (antivirus, adaware, etc.), but it's less than a third of what I used to use before I gave up and decided to go the way of regularly reinstalling... when your ready/organized for it and don't dread it, it turns into an easy ~2hour investment for a very lean and clean machine (I have always gone with two hard drives: my primary HD I always treat like I'm going to wipe next week, and my 2nd HD I store things I want to keep, valuable data, music, latest drivers, work/family/school related data, AHII downloads, etc.).