Truestory(TM)
I went from 32bit Vista (because my PC is old and dell wanted to charge me an extra £50 to not have Vista and stick with XP at the time...bastids) to 64bit win 7. It took me four different versions (nicked, borrowed or bought in various phases) to get one that would install properly.
The real problem? I dualboot with Linux. I'd already switched my jurassic copy of Fedora for Backtrack. It was only later I realised that windows gets grumpy if it isn't the first OS to get an install.
The irony? I only really use windows for gaming (mainly minecraft but also EvE online) and more and more minecraft texture packs and mods were ditching 32-bit. The derpy lengths one will go to to keep up with minecraft ^_^