Really low frame rates regardless of video quality settings. All advanced options are turned on except for vsync and AA. Doesn't matter what resolution - I get the same performance at 640 x 480 as I do at 1280 x 1024. No overclocking. Game was fine on my 3800+ single core and 6800 Ultra. Hell, it ran better on my Athlon XP 3000+ and 5900U.

Relevant specs:
CPU: Intel Core 2 Duo E6750 2.66 Ghz (stock)
Motherboard: Asus P5B
Videocard: PNY 8800 GTS 640MB
RAM: 2 GB Mushkin PC2-6400

It's also slow to save pictures off the web - takes a minute for the dialog box to appear. And I was ripping CDs simultaneously the other day with my backup computer, and the backup computer was faster than my main computer most of the time. It wasn't the same CDs, but the majority of the time the A64 single core was faster at encoding than my 6750 dual core.

Any ideas? The gaming issue is the main one. Could it be my mobo? It was a Craigslist purchase, used. Never had a BSOD on it, though, or really any problems with it.

Edit: HL2 and episodes and Portal all run fine with high/max settings. Doomsday engine with environment, weapon, and polygonal weapon packs runs fine. Nexius runs fine. Maybe it's a setting in D3?

Id say its time to reformat your OS
.Or try defragging the HD.Cleaning out temp files and registry.Run spyware cleaners.Also down load crapcleaner.It will clean out you temp folders and reg etc.and its free.There is now way Doom 3 should bring that system to its knee's

http://www.filehippo.com/download_ccleaner/

Update your graphics drivers, it could be some weird issue with OpenGL, which I'm told Doom 3 uses.

Had the 175.16s, updated to 175.19s, no change. Running a smart scan now. Maybe I'll revert to the 169.21s, those seemed to work pretty well for me.

Adaware and Spybot both found nothing. I keep it well defragged, and double checking right now says I have 1% excess fragments. This install was done in late March, so it seems like it should be fine. I generally try to reinstall every 6 months or so anyway though, so maybe I'll try that.

Be sure to download the latest Doom 3 patch. The latest patch offered the Multi-threading capabilities and compatibilities with many Dual core cpu's. If that doesn't work then try re-installing the game.

OK, uninstalled/reinstalled, installed the 1.3 patch before I played at all, same thing. This time I installed from the disks; the last time I had copied the disk contents to my hard drive and installed from there (much quicker).

Averaging around 20 fps although it does hit the cap, seemingly randomly. ergh.

edit:
Went here:
http://www.realtech-vr.com/glview/download.html
and ran that, it's an OpenGL version checker/render test, no problems with it. Ran DXDiag, confirmed running DirectX9.0c (D3 tries to install 9.0b). Since other games are OK, and it's run fine before, and everything else appears to be OK, I'm kind of stumped.

I have the exact same issue with UT3 and Grid. Not sure whats going on. Never had this with XP. Just loaded Vista last week.

Any thoughts for either Reason or me?