Hi newcomer and welcome to the Tom's hardware forum.
Ur question isn't clear with the title of the thread.
1- Do u want go back to ur old Windows XP installation?, sorry but that is only possible with a clean XP installation
2- Do u want enable the XP mode on Win 7?, check the Microsoft support page to know how do it.
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.