I'm not up to speed on Vista. I would think that you can, BUT knowing mickysoft,
both programs will want to control the system. You could probably do it by partitioning your hard drive and installing XP on D: part. and then install Vista on C: part. You might have to edit the .ini file in the root directory to accept either program at boot and specify/choose the default OS and how long of a wait to make a decision on which OS to choose (on boot, you'll have to choose an OS). You'd have to ask someone who's more familiar with Vista.