you could use the software for 30 days as that is the activation grace period - if you then try to activate it after this time using the CD key of your friend then an error box will come up telling you that this key has already been activated and asking you to enter another key in
One more question i do have about it, i herd that microsoft can tell if you have installed a pirated copy of XP. is this true? because i would like to borrow my friends copy and burn it and run that before i go out and spend an arm and a leg on something i might not like. but of course i dont want to get into trouble.