I think the EULA allows one installation of the OS on a PC and on installation on a Laptop. Not too sure though...
...but I know that you can't install Windows XP on more than one PC using only one CD. But it also depends on the licensing.
There's nothing stopping you from installing Windows XP on one CD to more than one PC however. It's just the law that's stopping you I guess
. If you were to do this, then you'd probably be unable to benefit (if at all) from Microsoft Update, but you'd still get all the security patches. No big problem.