Unlike previous versions of Windows, the upgrade version of Vista requires you to have the old version of Windows installed on the computer. Pre-Vista you could just produce the old version’s license key or install disks as proof of ownership. This can be annoying for those (like me) who prefer to do clean installs on a newly-formatted hard drive. Fortunately there appears to be a workaround, as described in the linked article.
In addition, installing a Vista Upgrade version over top of an XP system will invalidate the XP license key, and it will no longer pass the mandatory Windows Genuine Advantage check, should you try to re-install WinXP in the future.