Two ways to do this,see
HERE and
HERE -
Method 1-
a) Insert the Vista DVD - click "Repair your computer" - select your OS - click "Start up Repair" - now booot into Vista
b)Then download
EasyBCD - Add/Remove Entries - Add an entry - select Windows/XP
Change the name to “Windows XP”, then click “Add Entry” and “Save”.
Method 2 -
Use
VistaBootPro (you need .Net Framework 2.00 on your XP system to install this.
If you haven`t got that, or can`t be bothered to download that,use method 1).
Open VistaBootPRO, ignore the prompt to backup your BCD -
System Bootloader tab - select "Windows Vista Bootloader" in the first section
and "All Drives" in the second section and then click Install Bootloader
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".