Bill Gates makes sure there are not any easy ways to bootleg a copy of windows since W/XP but the XP versions seem to be a lot looser on the verification these days. Typically every copy of windows is only licensed to one machine and they poke around your machine for MAC IDs CPU IDs and some other unique identifiers when you authorize it. Lately I have noticed XP is not picky when I move it but I am sure 7 and 8 will be.
