Strange - I wanted to upgrade from XP to Win 7 Pro. There is no direct route so:
Apologies ahead of time to Miles for the off-topic reply, but....
Carol, MS in their infinite wisdom have decreed that no version earlier than Vista can be upgraded through the installer. This has to do with the new upgrade routines that rely on a WIM-aware OS being the one being ugpraded. That's what makes the upgrader do what it does so well.
Unfortunately, at this point in time the only other WIM-aware OS besides Windows 7 is Vista.