When my girlfriend got a new computer I went through the whole reinstalling XP from scratch process on her old computer so we could give it to a friend (so they had a completely clean installation). It wasn't difficult per se, but there were several issues. The main issue was that we have a WPA wireless network in our house and, after reinstalling XP, we couldn't connect to the network (since it wasn't until SP2 that XP supported WPA). This meant having to disable encryption on our router, downloading all the XP patches, and then re-enabling WPA.