Upgrade W2K to XP-Pro

I recently bought a copy of XP-pro Full because I intended to rebuild an old PC for a relative as a gift and wanted to provide the best possible OS. However, he has just bought a different system. So now I'd like to use this XP-pro on my Dell Inspiron which currently has 2000-professional. According to the MS compatibility site, I should be able to do an upgrade from w2k to XP without losing my data. However, when I start to run the Install CD it says "you cannot upgrade from Windows 2000 to XP-Professional". If I have to do a clean install, that's ok, but I'd rather do an upgrade so as to retain my data, etc.

Advice appreciated.

jo