you Can have a duel boot of XP...
You would need the disk for vista (OEM or other) to go in the WINRE (by booting to the DVD than going to repair computer than next), at this point you would need to open Command Promp't than type in diskpart. once you have disikpart type in list volumes (this will display every volume on computer including the vista). once you find vista volume type in select volume (volume #). now type in "shrink querymax" (what this will tell you is how much you can shrink the volume). now to shrink volume type in shirnk desired=(amount in MB). When your done shrinking the volue list the volumes once more to see if you new volume is their.
At this point you can install XP on other volume (XP may require an install over vista Partion and if so you the vista disk with step provided and install vista on new volume) but you may still need vista disk just in case any boot sector was damaged. if your unsure how to do this i would suggest calling microsoft for support or taking to a local shop.
I would suggest try vista ultimate because the compatbily with programs is very high. By the way what kind programs are you having issues with?
best of luck you jrwright