illegal windows XP

[Burnman]

My friend purchased a Compaq desktop around 14 months ago and had had no problems until recently when after a microsoft update it started to tell him the copy of windows XP was illegal. After a bit of digging it appears that the machine was bought with a XP home spec but the shop or Compaq had installed XP pro. He has a valid XP home key sticker on the desktop but because the installed software is XP pro it is telling him it is an illegal copy. Compaq and the shop where he bought are being difficult and saying it will cost him to have it fixed even though it is their problem in the first place, has anyone encountered this problem before and is there a simple fix or is a clean XP home install the only answer?

[macko72]

The only way to change from WinXP Pro to WinXP Home is to
format the drive and start over. There is no supported downgrade path
or technique.

Simply boot from the WinXP Home installation CD. You'll be
offered the opportunity to delete, create, and format partitions as
part of the installation process. (You may need to re-arrange the
order of boot devices in the PC's BIOS to boot from the CD.)

DO NOT FORGET TO BACKUP IMPORTANT DATA YOU MAY HAVE!!!