Possibly the wrong forum for this discussion, but,
If, in your BIOS, you have the CD-ROM set as the first boot device, then you put your XP CD in and reboot, you should go in to the XP install process without Vista ever booting up and you should have no problems. If you let Vista boot, then put your XP disk in and try to start the XP install process from within Vista, then you'll have problems.