Computer's video terrible suddenly

[natep]

(reposted from General Hardware, didn't get any replies)
I built a computer for a friend a couple of months ago. It was running in tip-top shape until all of a sudden, his graphics are slow in Windows browsing, and games like HL2 and WoW wouldn't open. Here are the specs:

Shuttle SN25P
A64 3000+ originally OCd to 2.5ghz (prime stable for 24 hours)
1gig Corsair Value (passed Memtest after the OC, twice)
Gigabyte 6600GT PCI-E
Samsung 160gig SATA HD

The video card is apparently still working somewhat, because there are graphics on the montior, but FireFox and Explorer scroll slowly, almost like it moves in single frames or waves. Windows loads quickly and normally, the computer can open AIM, browse the Internet, and type in OpenOffice. WoW and HL2 crash on startup however.

I've replaced the SATA cables, reset my OC back to default, had him run Ad-Aware and AVG scans, and installed new video card drivers. What else should I have him try, or what do you guys think could be wrong?

 

[ChemGeek]

Is it as if something is running in the background and using up the CPU time? Was the video card overclocked?

 

[natep]

The video card isn't OC'd. I'll ask him to look in task manager and see the CPU usage.

 

[ChemGeek]

One other thing is to try and run the adware scan and the antivirus scan while the comuter is in safe mode (hit F8 while booting). You might have done that already I am not sure.

 

[sintaxera]

Run your CPU at regular speed (just in case) and go to Windows device manager. Make sure your system is using Nvidia drivers and not standard VGA/default adapter drivers. Maybe, just maybe, the system will need to be forced to use the Nvidia drivers if they refuse to install using the normal set up routine. I've only run into this a couple of times though, but it's easy to check for.

 

[natep]

Thanks for the suggestions, anyone else have anything?
I'll get post after I try the Ad-aware and AVG in safe mode, and check the drivers.

If those don't work I'm going to try a reformat and fresh Windows install. After that, I'm afraid it will probably be an RMA back to Newegg, or something along those lines.