well i found this...
"Upgrade versions only:
4b. Insert qualifying media
If you are attempting to clean install with a Windows XP Home or Pro Upgrade CD, you will see this screen, which requires you to insert your previous Windows CD in order to verify that you qualify for the Upgrade version.
Curiously, it says that you can use CDs from Windows NT 3.51 and Windows 95 in addition to those from 98, 98 SE, Millennium, or 2000, though these products cannot be upgraded to XP.
Once you've proven that you qualify, hit ENTER to continue."
So I guess I will take his word for it...