Slow framerates in everything

Heres my system specs:

P4 3.2 HT
Gigabyte Geforce 6600 GT 128mb
2 gig pc2700 ram
80 gig SATA HD
Windows xp sp2
onboard realtek sound

My problem is every game I try to play, I get choppy perfromance. Ive tried all the latest drivers for everything. When I play counter-strike, when I first join the game, for about 1 min my frame rate is silky smooth and when i type timerefresh in the console it will read around 200, everytime anywhere. Then over the next 2 mins ill notice the game getting choppier and choppier until when i type timerefresh ill be getting 50-90 and the game will be very choppy. I need help!