I beleive XP has a check somewhere that you can invoke, and this will confirm that your software is valid or not.
The "Tools / Windows Update" process now includes a check called "Microsoft Genuine Advantage".
If you get a message "No updates available" or the tool downloads the latest updates, then you know that Microsoft see your copy of XP as legitimate and licenced.
Otherwise you will see a message telling you that it is not.
RC