there is, but if you're buying THAT, you might as well get Windows 7 anyway.
if you get win 7, buy the family packs that allow up to 3 computers per registration.
It comes with the upgrade CD, but here's the trick: Install windows 7 from the upgrade CD. Skip activation.
reinstall, choosing upgrade, activate it this time. This is NOT a cheat and is how Microsoft intended it to work.