Their call-in support for the activation is actually pretty lenient. When XP was still fairly new I had to send in my laptop for warranty repair. During this time I borrowed an unused computer from home, installed the same copy of XP on the temp machine, called them up and even though not a single piece of hardware was the same as the original installation, the guy on the phone just asked a few basic questions like how long I would have the temporary machine and whether I owned it.
Not all that shocking, but what surprised me was when I received my laptop back, I had to reinstall XP (it did not come with the laptop and they ended up replacing my hard drive) and the Activation did not complain even though the key was simultaneously in use on my temp machine.