And it couldn't even be demonstrated at the auction. You might need to know what SP was loaded, ie SP2 or SP3. I don't even know if that matters but hopefully someone will be here to tell us.
I've got XP Pro with SP2, and it's my own copy, not a M/S original. It's authentic so you'd have to use the COA that's attached to your brother's PC. Find out what you can first about what was on his PC.