I have installed and use my win XP pro on several machine already and found no problem with it. just give it a try though and post here any error mesage or problem you will encounter if you will install it on several machine. just dont register for Mr. Bill not to track you down that easy.
If you have a branded machines with the same company. say Dell, you dont have any prob installing it on several Dell PC.
Anyway, just give it a try. You aint ganna lose your bucks since you can easily crack that.