I had the exact same problem on my Vista 32bit system, turns out the GPU was buggered, had to RMA it back to the shop and got a replacement which worked fine.
Havent a clue what it was as all i did was replace the GPU and worked fine, didnt even reinstall the drivers, just plug and play.
Was a Palit GTX 470 btw, which are god awfully manufacted crads, one of the fan fins broke off, twice! (second tiem i got the above problem) so replaced it with the Asus GTX 470 and No problems since.. But have upgraded to Win 7 64bit thou