Hi kemical,
I have now downloaded a new copy of the Vista 64 bit ISO from my university and I know that this one should definitely work, as a friend has downloaded it before and it worked for him. But after downloading and burning to DVD, suprise suprise, it doesn't boot! Unfortunately I don't know anyone else who has a Vista disc (they all have it pre-installed on their machines), so there are no more discs I can try.
Do you think the problem could be with the DVD drive itself? It booted from the 32 bit XP disc (which is a CD) fine but just doesn't seem to work with these Vista DVDs.