Hi j.ingham06!!
I hope you already created the Recovery Disc using vista before downgrading. Because if you haven't do so, you might face difficulity to upgrade to vista again in near future (or maybe far future). Furthermore, your
Recovery Partition won't be working after you downgrading
Well, this is a link from HP for downgrading from Vista to XP, eventhough its only a guidance
Changing the Operating System from Windows Vista to Windows XP
Hope this helps you a bit