Ok, work at a school, and last year we heard about a kid that was doing up computers and selling them, and someone said that he had nicked that key's off the side of the workstations in a few of the labs.
The kid probably got his hands on an academic or corporate version of Windows. I picked up a copy of XP from a local university and the installation lacked the authentication process usually seen at the end of the install. I could register XP, but there was no authentication like what you'll see on a retail copy. You could put it on as many computers as you like and not have a problem with updates (I know one guy who does!).

I've also seen both academic and corporate versions of W2K Pro that did not even require a serial number. If you're eligible for any kind of academic licensing, and more people are than realize it, take advantage. No problems from what I've seen getting updates on any of these versions.

A few years back, I did an informal survey of techs during support calls. At the end of the call, if I had good rapport, I'd comment they obviously knew a lot about computers and could I ask them a personal question:

"Did you pay for all the software you ever used on your computer?"

I asked this question weekly for about a year. I never heard one tech say "yes". Half of them refused to answer though.