You cannot downgrade from any higher SKU to a lower one just by using a key. You will need to do a full reformat since there's no option for downgrading, only upgrading (via Windows Anytime Upgrade). You however can still save profiles and settings by using Windows Easy Transfer, but unfortunately, you will need to reinstall all softwares you're using.
Hope this helps.
you can with 2 HDD's, install XP on the other HDD, disconnect the HDD with Win7 and have only the "clean" HDD hooked up, install XP,shut down the computer and hook up both HDD's, reboot, the "impossible" done again