Vista: Can neither uninstall PT 7.1 or install 7.4

[j12d20v71]

I got the notice yesterday that ProTools 7.4 is now available and is compatible with Vista.

So I downloaded 7.4 but figured I would wait to install it until I had installed Vista. Once I installed Vista, I tried to install 7.4, but it wouldn't let me because "another version is installed (7.1) - please remove 7.1 before installing 7.4". But I couldn't uninstall 7.1 because "Windows XP w/ SP2 is necessary to uninstall 7.1."

So I look up help on how to uninstall Vista. It's not possible: "You cannot uninstall Windows Vista, but you can reinstall your previous operating system, which then replaces Windows Vista. To reinstall your old operating system, you must have the original installation disc. Back up your programs and files, insert the installation disc into your computer, and then reinstall the old operating system."

So I get my original Disk and try to reinstall XP but "Setup cannot continue because the version of Windows on your computer is newer than the version on the CD."

I'm stuck. Any suggestions?