Mysteriously slowing 275 GTX

Hello All,

I'm stuck with what's wrong with my stepson's PC. I built it for him a few years ago, and it was a low cost system since the games he played were not graphically demanding. He has since upgraded the video. Here are his current specs:

Pentium D 820 Dual Core
Gigabyte GA-945PL-S3
2GB DDR2
1k watt PSU
Hitachi 7200RPM HDD, 1TB
EVGA (I think) 275 GTX
Win 7 Pro x64

So, right off the bat, I know this isn't what you'd call a cutting edge system. However, since he spent 90% of his time playing Counter Strike Source and WoW, it seemed more then adequate.. and was. Until recently.

About a month ago, he said he was having slowdowns in WoW on raids. I don't know much about WoW since I don't play it but told him to turn down some settings. It still lags behind quite a bit. Now, even games that worked fine previously are lagging badly. Counter-Strike Source was freezing up today.

Everything looks fine to me. It's a pretty fresh windows install, there is no spywware that I can detect, and he doesn't have excessive processes running. His DX is up to date, as are the Nvidia drivers.

Any ideas at what I should be looking at?

Thanks