I found this out in a predictable way, at Google they had machines that they bought for desktops which came with an OEM windows sticker but they always put Goobuntu on. I needed an XP license for a project so I thought, "Oh I'll use this one, I know its not being used elsewhere." and did an install with a vanilla distro and the product code.

The first thing that broke was it wanted the oem cab file. Fixed that.

Then it would complain "this product key is intended for the original system."

Since I was running in a VM I changed it piece by piece until it accepted the product code, first changed the type of CPU it was reporting, then the bios identifier string, and finally the ethernet MAC address. It finally acceepted the product code after all of that (not sure which parts were optional but only needed it for like a week) Considered those licenses pretty much worthless from that point on.

That's a pity. I just bought a new ThinkPad and there was no way to get it without Windows. It runs Xubuntu now and it's really snappy. I'd be happy to give away or sell the unwanted Windows license.

At least in Germany (possibly in the EU in general), that's no longer true. There has always been a market for OEM license resales, but recently there was a ruling that gave it a legal foundation. Microsoft wouldn't tell you it's okay, but they are no longer allowed to prevent it.