You can use it as many times as you like.

Put it on a machine that won't connect to the net.
Activate it.
Then put it on a machine that will be used on the net.
Activate it again, XP will think you've just rebuilt your PC.
Download all the updates as and when.

The only time you may have trouble is if more than one machine with a serial number tries to access the Microsoft updates.

I've tried the dodgy licence keys - Microsoft has pretty much identified them all, and won't let you download updates.

Posted By: Arganth, Jan 6, 10:05:26