My old HD crashed and burned which had Windows XP that was upgraded to Windows 7.
I built a new system and used the upgrade to install Windows 7 bypassing the Windows XP install.
Called MS to approve the install and they would not saying I have to reinstall Windows XP and then use the upgrade.
As the system is now my Windows XP disk is prevented from installing by the current Windows 7 install. And the Windows 7 will not activate as it was not intended for a "clean" install.
I currently have two HDs that are mirrowed in this new computer with the "up grade" Windows 7
I supect that I will have to reformat my hard drive in order to install Windows XP so that I can use the upgrade to go back to Windows 7 unless someone knows a better way.