just follow these easy steps.
1. Boot into GParted. link here:
2. I used three partitions - a 30GB for Vista, a 30GB for XP, and a 100GB for various backups in case I have to reinstall either.
3. Boot from the Vista DVD and install to the first partition. This'll be the C drive. Make sure everything works, Internet connected, etc.
4. Boot into the XP CD. Install it to the second partition, which will be the D drive (shouldn't have any problems there). At this point, once it's installed, it will have overwritten Vista's bootloader, so now you can't get into Vista.
5. Boot the Vista DVD again, click "Repair your Computer" when you get to the menu, then go to Startup Repair. It should scan your system and say something about corrupt sectors. Whatever. Basically it should overwrite the XP bootloader with the Vista one again, so now you can get into Vista but not XP.
6. Get into Vista, and download and install EasyBCD (Google it and click the first link). It should show you your bootloader when you start it. Click Add Entry, select Windows 2000/XP/2003 in the dropdown box, and it should go ahead and do that.
7. Reboot and test things. The bootloader should have entries for both Vista and XP loaded into it, and both should work with no problems.