I've been trying to find a dirt cheap copy of Windows XP or Vista recently. Not that I would ever want to buy a copy of Windows exactly, I'm looking for a copy for my sister, who can't get with the program.

I was very interested to read this article from ArsTechnica on OEM versions of Windows, which are about half the cost. I wasn't aware about the motherboard limitation.

Buying OEM versions of Windows Vista: the facts: "OEM software is also tied to the motherboard it is first installed on. Unlike the retail versions of Windows which can be transferred to a new computer, OEM versions are not transferable. What about upgrading hardware? Microsoft says that anything is fair game, except the motherboard. Replacing the motherboard in a computer results in a 'new personal computer,' which the company considers to be synonymous with a transfer. It's not permitted with an OEM edition of Windows.

Nevertheless, I've known users who got around this limitation by calling Microsoft and reporting that their motherboards died when they wanted to build a new computer. It is Microsoft's policy to allow motherboard swaps in instances where a system is defective or has suffered a hardware failure. But you shouldn't bank on this approach; there's no guarantee it will work and, well, liars don't get ice cream. Also, while retail versions of Vista include both 32-bit and 64-bit versions of Windows, OEM versions are specific. You get one or the other. This follows from the per device restriction. The end result is that OEM versions may not be that attractive to users who frequently build new computers from the ground up."