If you install XP on a system with Windows 7 already installed, then you will have to use a Windows 7 Installation disc to repair the MBR. When Windows XP is installed, it overwrites the MBR, but it's very easy to fix. Complete the installation of Windows XP, then use the Windows 7 disc and go to Recovery. Click the Startup repair and Windows will boot to Windows 7 which rendering Windows XP installation unusable.
After this procedure, install EasyBCD and add the Windows XP installation and that's it. You now have dual-booting on your computer.