Dual booting with preinstalled Vista is trickier than if you had XP already installed and wanted to add Vista. BUt here is what you need to do.
Make sure you have a second hard drive or partition to install XP onto.
Install XP on your second drive/partition. Once your system reboots it should automatically load XP. Do not panic and think you have lost Vista. It's still there but you need to fix the bootloader. To do this you will need a program called VistaBootPro, which you can get here:
http://www.vistabootpro.org/So install VistaBootPro while running XP and repair the Vista boot loader, like this:
Open VistaBootPRO, ignore the prompt to backup your BCD. Go to the Bootloader tab and "Reinstall the Vista bootloader". Like this:

Next, go to the Diagnostics item on the menu bar and select "Run Diagnostics". VistaBootPRO will default back to the Manage OS Entries page and you will see that you now have entries there for "Earlier versions of Windows" as well as "Microsoft Windows Vista. Here you can change the default OS, edit the boot timeout period, and rename "earler version..." to "Windows XP" if you want to.
Source:
http://www.pro-networks.org/forum/about88231.htmlNow reboot!
Now you should see the dual boot options!