That's funny. At Fry's, they sell tons of OEM copies of Windows to Mac owners - they just make sure you buy some kind of PC computer hardware along with it.
After doing this for 3 Macs, I now have quite the collection of case thumbscrews!
MS actually came down on Fry’s a couple of years ago for this. I believe the minimum hardware that you have to purchase to count as a “rebuilt” machine is either a hard drive or memory.