Hey Jon
Sorry to here you are having problems with Vista. It is possible to revert to your previous OS, but your Vista has to be bootable. Use the DVD and try to repair the damaged part of Vista. I am at work now, but I will come back later on this evening and give you the step by step processe to uninstall Vista. But first use the DVD to repair, even if it has to reinstall it, do it. Your files from XP are not missing just archived in Vista, so there is a way to revert back aslong as you can boot into vista.