If you have a restore CD,

you may NOT be able to install XP on a different machine. If you have a Windows XP installation CD, you can.
Microsoft will allow you activate your XP on different machines for several times at unknown intervals. You can always call Microsoft to activate your XP. They just want to make sure there is only ONE copy installed on ONE machine at any time.
What Microsoft does in activation is to simply discourage "causual copy".