NV_4 Disp BSOD error

first of all - any help would be very appreciated - this problem has caused me lots of problems over many months now
The main problem is that im not even sure what the problem is.

2 years ago i built my own computer as follows:

Asus P5B mobo
intel core 2 duo 6600
1 GB corsair ram
Nvidia Geforce 7900 GTO

having worked perfectly for almost 2 years (with one major problem with the PSU blowing out rather spectacularly with blue flames....) the system seems to be failing on me. I first noticed the computer slowing down and seemed quite sluggish especially when movies were playing. I then bought assassins creed and got a whole host of graphical problems which made it unplayable. I tried every possible fix known to man - including driver updates, virus/spyware checking and dusting the insides. I then reverted back to a few old games, namely dungeon keeper 2 and hitman. When playing these i again have terrible graphical problems - even with just the start menus. Dungeon keeper 2 is almost playable but freezes every few seconds for about 5-10 seconds. My computer cant even cope with the hitman menu for some reason. Ive since then tried playing a friends version of sins of a solar empire - a newish game which is more suited to xp but without the huge demands of assassins creed. It stops and freezes right at the start when the opening videos come on. Then up comes the blue screen of death saying nv_4 disp....something about an infinite loop. Now this error message has come up many times in the last 3 months and usually when videos or games are playing.
Can anyone help? is there a utility to test my graphics card to see if its faulty?
Any help appreciated

 

ah and should have mentioned im on XP w/SP2