COMPUTER KEEPS CRASHING, PLEASE HELP!!!!!

I have a home built system that i made solely for playing high end games. its not the most powerful computer out there but it was good enough to play on crysis when it first came out and run Call of Duty World at War at the highest specs.

then it all started going wrong

about a year ago now, whilst i was playing COD WAW the entire system started crashing to a BSOD saying that there was a BUGCODE_USB error with a lot of code at the bottom which used to say 0x00000002, 0x88e54040 and after trying to fix the problem went to 0x000000fe (0x00000002, 0x88e66738) and then rebooting. this only happened occasionally and then got worse and worse and worse, until i couldn't actually play the game at all because when it crashed all the progress i had made in the game wasn't saved.

at that point i was leaving the country for a few months so i didn't bother fixing it. but now i have arrived back and COD Modern Warfare 2 comes out in a couple of weeks and i don't want the same problem happening, so i have been trying to fix it for the past week.

i have totally wiped and re installed the entire system - it did nothing
i updated all the drivers for usb related devices - it did nothing
i removed my wireless dongle (as i read that this might cause some sort of conflict) - it did nothing
i ran memtest for an hour to see if there were any problems with the memory and it said there wasn't
i switched the RAM sticks into different slots and different configurations - it did nothing
i finally started removing the usb slots from the mother board, this did do something....

i no longer go to a blue screen, but the whole game locks up, the same sound is played over and over on a loop of about half a second, and eventually i am left with no sound and the screen frozen on the last thing i saw, leaving no option but to hit the reset button.

THIS IS DRIVING ME INSANE!!!!!!!

i have tried everything i can think of and any help would be so much appreciated, i cant take this any more and i don't want to have to give up on this system as it has worked so well in the past!

at one point about a year ago, it would also go to the BSOD when i was watching movies, but after a reboot, it stopped doing it, and it only happens now when I'm playing high end games, maxed to their highest graphics level, if i drop them right down i can play, but what's the point in having these amazing games and this powerful computer if i cant take advantage of it!

Heres my system specs:

ADM Athlon 64 X" Dual Core Processor 6000+
2046MB RAM (on 2 sticks)
Running windows XP 32 bit Home Edition
NVIDIA GeForce 8800 GT
ASUS M2A-VM HDMI
2 Hard Drives:
1 at 32 GB for storing my documents and other important work files
1 at 250 GB partitioned into 4 parts for separate workings (one of which is dedicated to games, but this shouldn't be the problem as i did not have it set up like this at first)

ANYTHING would be great, so please get back to me, i have no idea what else i can do.

Thank you