As far as I know, all Dell machines have had a license sticker mounted directly to the case. This would certainly constitute proof. Personally, I'm of the opinion that it'd be up to M$ or whomever to prove that the machine didn't buy a license at some point. As long as you aren't installing one OS disk on all your refurb machines (which'd be a big no-no). Besides, nobody sues people with no money unless they're complete morons or the RIAA (sorry, repeating myself).

-Zeke