If you have a single user licence, you can install XP on your mates PC, so long as you get rid of XP from your PC. If you keep yours and install it on the other computer, then you officially become a criminal.
So, theft is a misdemenour. How much do you need to nick for it to become grand theft, a felony charge?
Of course, if you want to be a Good Boy, you'll probably have to phone up MS to explain what you want to do. As a 'foreigner', I've found them to be a bit of a pain in the posterior (their's a language barrier despite the fact we're supposedly speaking the same language - odd, huh?). Good luck.