I use AVG and Ad-Aware.
Anyway, I went out and spent $100 on fans. Threw in another 120mm case fan into the front, replaced the stock Intel CPU fan with a Zalman CPU fan/heatsink. Also cleaned up the cables. At first boot, my CPU was running at 21*. After playing F.E.A.R. for half an hour with all settings maxed and getting over 50 FPS the entire time, it was at 35*. Needless to say, the temperature was most definitely the issue.
Oh, and the results from the Counter Strike Source benchmarks - a constant 115-118 FPS!
I uploaded a photo of my rig after installing the new fans and cleaning up the cables, so you all can check it out.