Probably the biggest hassle was having to phone Microsoft to reactivate Windows three days later, after I was sure it was running OK, and that wasn't too bad.
I managed to spot the trick question: 'How many PCs do you have this software installed on? Press 1 for a single PC, 2 for more than one PC."
Oh, and I went from an Intel CPU and chipset to a newer Intel CPU and chipset, which I imagine is easier for Windows to cope with compared to going from an AMD to an Intel system or vice-versa.