A few tips:
1. Before you do anything, back up any important files on the computer. This will also erase your XP installation.
2. Download the x86 version, not the x64 version.
3. When you run Boot Camp Assistant, make sure to make the Windows partition at least 30 GB.
4. Open XP, then insert the disk, and the Vista installer will start automatically. Don't try to boot from the Vista DVD.
5. The computer will restart several times during the installation. It's important you stay with the computer the entire time, because when it restarts, you'll have to press OPTION, and choose "Windows" to continue the installation.
6. Very Important: As soon as you can, right click on the Vista Desktop, and click Personalize. Set the screen resolution to the setting all the way on the right of the slider.