Vista Licensing
You can do the upgrade your existing XP which will wipe it out or your can click custom installation, repartition your hard drive and install vista on anotha partition, so you have both in dual boot,
you are fully supported because XP and VISTA use differant activation and serial numbers, and you payed for both of them.
if youre using 64 bit xp you will need to use a bootable partitionmagic disk to resize your partition if you installed xp on the whole hard drive.
this is how you do it on retail vista normal one, i dont know about the upgrade one, it mught have custom button on the installer or it might not, probably not though