I upgraded my computer yesterday and Windows is mad because my motherboard is different [the danger of saving money with an OEM version], so now I've got three days to get a new copy of Windows before I'm locked out. I was already planning to get Windows 7 64-bit sometime in the future, but I was going to wait a little while to do that.
I may still have a 64-bit copy of Vista around somewhere, but I'll have to dig around for it.