Open Question: computer screen freeze when playing games?

geforge 8800 gt had this computer for 3 years with no problem but 2 months ago when i was playing a game, the screen froze and i saw pixels of green and purple in a few places. i just turned off the power and turned it back on and it worked fine again. tonight i was playing a game it did the same thing but when i turned back the power and i started up the game again, the screen froze again and i saw alot more green and purple pixels. finally, i updated my video card at nvidia website and this time when i play any game, the screen freezes, goes all black, then goes all white. it stays white untill i turn off the power. i cant play any of my games since tonight :(