XP can and often does refuse to boot when the hard drive it is installed on is moved to another motherboard.
I had this problem and despite trying numerous solutions i was forced to reformat and reinstall. Ironically this was probably the best thing as I started a new machine with a clean install (never a bad thing).
What can you try.
Boot the machine with the XP CD in the drive and when it boots choose the recovery option.
If this does not work you can try to reinstall XP over itself.
Have a look at this http: // support.microsoft.com/default.aspx?scid=kb;en-us;315341&Product=winxp take the spaces out each side of the //