Well,thanks for the not totally bad news everybody! To be fair, a lot of suppliers are advertising these Dell discs, sealed and with unique serial numbers, and are all at pains to point out that they will provide a legal, working and updatable copy of XP on any one computer. If that is not in fact the case, then I am sure that their feedback would show this, or you-know-who would have removed these items from the site because of complaints. Anyway, the only way to find out is to build the new machine, bung the disc in and see what happens! I'll report back if appropriate.
P.P.