Well, it is technically not legal to have the Windows OS installed on any computer without a physical license.
However, you may be in luck - meaning if you have a branded machine from the last 6 years or so (dell, HP, compaq ect...) then you may be able to use the manufacturer's keycode. This keycode is the default one the OEM's use when building images for their machines. You can find it here :
or the wiki page :
The next trick you could do, though I don't know if it works with any other manufacturers, but Dell I know it does. If you have the dell windows xp re-installation cd you can just pop that in, run through the prompts and it will not ask for a key code (it's embeded in the CD and checks the hardware for oem compliance).