first installl XP and then VISTA QUITE SIMPLE
and then dont start anything for which the system files go wild and you have these problems , so just reinstall both.
First Xp and then VISTA buT keep in mind that Vista is not on that path like XP so many problems still exist in it, why dont you use XP only.
its ur wish so if you want to install both then XP then VISTA
I TOO HAD VISTA ULTIMATE x64 before but removed it and now i am happy with Xp only.