You can only install one copy of OS or Software eventhough you own it, on your systems. You are required by MS to have 1 License per machine for app or OS. Some vendors differ on that, but for MS's sake, it's a per machine thing.
They actually offer a multimachine licensing option for home use. When you can make lots of money, why stop?
if you purchase a new version of an OS ie, XP from 98, 98 becomes free and clear from the previous license terms. You can have that one version on as many machines as you want. I can't recall where this was found, but I believe that Groklaw
has some data on it somewhere. This only works for home environments though as far as I understand it. So the enterprise still has to have running licenses for all versions.