Hi Moe, welcome to NST.
Follow some of the early links in the sticky thread, and you should find the information you need.
(For Vista you can read W7 - they're functionally equivalent as far as dual-booting goes)
1. Boot from the Win XP CD.
2. Follow the on-screen instructions to install XP to a separate partition than Win 7 (resizing and creating another partition if necessary).
3. Once XP is installed, you will only be able to boot XP (Win 7 will be inaccessible).
4. Install [thread=642]EasyBCD 2.0 Beta[/thread] in XP.
5. Use EasyBCD | Manage Bootloader | Reinstall the Vista/7 Bootloader | Write MBR. This will replace the XP MBR/PBR with the W7 versions, like it used to be before the XP install.
6. Add an XP entry under Add/Remove Entries | Windows tab, and let it auto-configure when it offers.
7. Reboot, and you should have a dual-boot.