Remember Windows 7 & Vista have additional services not found in XP by default. For one I disable the search service and a couple of the WMC ones, like the media network sharing.
I went back to my XP x64 image after several months of Win 7 only to make sure I'd be able to upgrade and couldn't bear it. But it will probably be dependent on your specific situation & environment. Some drivers
& software will be up to spec, some may not.
Edit: I did a new clean install of XP x64 and let it activate & update - it sounded above like I restored an old working version of XP.