not that this is underclocking the GPU. (make sure you dont accidentally overclock)
if that helps then its clear that your GPU is faulty because its not able to operate at the stock speeds. if so ==> RMA time
But first try it in a different computer, fresh install etc.
strange still that it crashes in winXP but not win7. my nVidia card does it the other way around.
well, all the much more reason to get win7 :P
well the reason i installed XP again is that, win7 is very incompabile