xp on 2 systems

ok well i'm buying a new computer soon and i really dont' want to go out and buy another OS so i was just gonna use the xp disc i already have, but the thing is that the computer i'm using atm my grandfather wants to put it in the garage ( he likes to work outside and wants to get online for instructions on fixing things) so i told him yeah sure i'd just install like a wireless pci card so he can get online out there. I know your not suppose to have an OS on 2 diff. systems but is there anyway i can do this w/o having to buy another xp disc, or going to jail.

 

Install linux on one machine instead of xp, or buy another xp copy.