Multi-boot pre-installed system? That's a first...
Use a Vista disc or download and burn
the recovery disc from this site to perform a startup repair. You'll know if Vista is installed or not depending on the fact that an installation should appear in the list of systems to repair.
Look here for information on doing a startup repair of Vista. When Vista is booting fine again download and install EasyBCD. Add an entry for XP and you should now be able to choose between XP or Vista at boot time. Post back with a disk management screenshot, boot.ini, and easybcd's entry output if you are still having problems with getting the XP entry to work.