Try all the different PCIe slots.
Also, have you tried booting into safe mode? (could still be a left over driver conflict). If you can it suggests a driver issue. If so you can use easyBCD ( http://neosmart.net/dl.php?id=1
) and set "Run in SOS mode" to see a detailed boot log.
You could try booting with your windows CD and doing a repair. (or if you have time to waste, do a complete reinstall of windows :-D)
Sorry, I'm just guessing since I really have no clue. I've had USB devices and 3.5" floppy drives stop windows from booting before and removing/replacing them fixes the problem, but since you have it working on another PC I'm sure the GPU is fine.