Try using a slightly older driver for the video card.Sometimes the newest drivers are not always the best to use.Let me know.
C2D E8500 STOCK
2X2GIG DDR2 1066 IN DC MODE
TOUGHPOWER 850WATT PSU
EVGA GTX 260
SMILIDON RAIDMAX GAMING CASE
LG 24IN WS LCD 1920X1080
250GIG HD/500GIG HD
G5 GAMING MOUSE
LOGITECH Z-5500 5.1 SURROUND SYSTEM
500WATS CONTINUOUS,1000 PEAK
WIN XP PRO
Hmm... I would uninstall your video drivers completely. After you do it, restart your comp and THEN install the drivers again. If it still randomly restarts, check to your gpu temperature with the riva tuner, might be overheating for some reason. If that doesn't work, test your graphics card in another computer and see if it runs stable. If it does, then you have another problem on your computer my friend (could be your psu, mobo, might have to reseat your cpu, make sure to run a memtest as well...). Thats all I can think of right now.
1. Uninstall drivers, restart, resinstall drivers
2. Check your GPU temperature using nVIDIA Riva tuner
3. Try the card in another computer, if it works, step 4!
4. Assume there's something else wrong with your computer
5. Make sure you took off any plastic covering or anything that was on the GPU to protect it.
I don't think that is a overheating problem, cause it NOT randommly restart...
I can play to CRYSIS all settings to high during an hour but can't open the nvidia control panel cause it freeze. Also, there some game that freeze before launching (freeze with de windows screen and not a black screen)
So i assume my graphic card is ok (playing crysis or fear 2 for hours)
memtest = no error.
Tell me if i got something wrong
Finally i will reinstall the driver for another time, it cost nothing to give it a try