i bought the xfx geforce 8800 gts 320mb ddr3 almost 5 months ago or so, been gaming with it and nothing really went wrong until recently...i play mostly world of warcraft, and the game would freeze for about 4-6 seconds before returning to normal, this happens randomly, sometimes i can game 1-2 hrs without it happening but sometimes it happens every 10mins or so...also i've been getting this error msg + blue screen of death...
you might try defragging the hdd,could be that due to 5 reinstallations that the files are scattered to hell and back ??
the only other course that comes to mind is reinstall windoze right over your current setup,also see if you cannot find some video testing software,like a long movie ,if nothing else shows up,one more thing is your gfx card running at normal temps,well cooled etc,?,.