Ok. First time posting, but I'm desperate for a cure.
I know this is a very general problem, but I'm hoping someone might be able to somehow narrow down what the problem could be coming from.

Computer Specs:
P4 3.00 ghz (w/ hyperthreading)
Asus P5P5l2 mobo
PNY Geforce 6800 GS (factory overclocked)
1MB RAM
Audigy 2 ZS soundcard

Problem:
Well this computer was working just perfectly about a year ago. Flawless, running Guild Wars, Battlefield 2, ETC at very playable frames per second. I had to leave it alone for about 3 or so weeks and when I started to use it again, it just felt terribly slow, even after defragmenting, memory testing, and cleaning out some useless programs. So I opted to reformat. Using Windows XP x86, everything seems fine but now Guild Wars and Battlefield 2 are still unplayeably slow. Pretty much any game I try to play is just painfully slow, except for Starcraft. I reformatted once again. Same issues. Now I'm utterly confused at what the problem could be.

Attempts at fixing:
In the device manager, I've found all necessary drivers so that there are no unknown drivers.
Reinstalled the LATEST directx files, as well as the latest nvidia drivers.
Swapped out the GeForce 6800 GS with my BROTHER's (exact same card). Same benchmark score as my ORIGINAL card. (My brother's computer is working perfectly with either his or my card so the card shouldn't be the issue).
Did memory tests, changed hard drives, looked around the bios (no problems I could see).

That is about all the things that I've tried doing, with no avail or even a slight increase in fps. I get the same benchmark scores with 3dMark06 -- 175X. The scores that most people are getting with similar computer specs is 2000 to almost 3000.

Any help at all would be greatly appreciated.

Forgot to mention, I'm using a netgear USB wifi adapter (I really DOUBT it can cause the computer to slow down, but I really have no idea)Open it up, clean it out.
Use this guide to help.
Also, check all connections inside the case, make sure everything that needs to be connected is nice and tight.
Re-seat components if necessary.