I have a conference call at 2pm EST to discuss Enterprise Agreements and Select Agreements with Microsoft.
You have 1 or 3 year terms, mostly 3 years terms. At the end of each year, they have a 'measure up' or some term they use, where they log all your systems and then anything above and beyond they pay a set fee for. Most of the time companies buy extra copies - more than they need.
So once they start going over that limit and realize someone might be using their key, they can request a new key after that for new installs which would make your current key worthless for validation..
Only on Windows XP though.. thought I'd share..