First go into your bios and tell it to boot off your hard disk with the working vista install.
then all you need to do is edit the BCD file on your hard disk and remove the entry for the incomplete install. eaither use built in microsoft utility start -> run -> Bcdedit.exe, (http://www.microsoft.com/whdc/driver/tips/debug_vista.mspx).
you may want to add an entry for the xp instalation at the same time, (alternatively try easy BCD for something more user fiendly.
If you instalalled vista on the same partition as the xp buildit a well have damiged the xp install and you might have to repair it or rebuild. if yo do an xp install / rebuild youl need to run the vista repair to restore your BCD file otherwise youl keep going into xp.