"But seriously, if you've paid for the licence you can put the OS on any single machine you like"
Not true.
If you buy a copy, it is installable on any machine. Pre-installed OEM versions are normally limited to the machine they were supplied on, even if you were lucky enough to be given a CD.
I bought a sealed OEM CD, including sticker, which was claimed to be usable on any machine. Microsoft refused point blank to authorise it on my machine.
You can use keygen to get around it, but that would be cheating...