I assume you have to mail in the coupon and the receipt to receive an actual physical copy of Vista? Shouldn’t be that hard to come up with a receipt, lots of places provide receipts that are REALLY easy to fake. Or just go dumpster diving outside your local Fry’s, shouldn’t take more than a few minutes to get a receipt for a new computer.
Still, why would you be buying a copy of XP MCE if you didn’t just purchase a new computer?
ON EDIT: The above probably sounded harsher than I intended. This is important information you are providing, and I’m sure many will appreciate hearing about it.