Leave the boot order the same as when you installed Vista.
Download the utility recommended by Colin several times, he says:
You can download this tool from
http://www.vistabootpro.org/intro.php. It
is free
Then you add a legacy system using the tool.
Copy the boot.ini and ntdetect.com and ntldr to the drive with the Vista
boot folder from your former system drive.
Modify the boot.ini to reflect the current position in boot order of your XP
system.
(you can play with it (if you can) or use the XP Recovery Console, bootcfg
to /add (your legacy systems) or /rebuild the boot.ini to get it correct.
This will give you a first boot with a choice between vista and the legacy
you added, and if you chose the legacy, you will get the boot.ini menu or go
straight to the xp system if you only have one system (I have several)
(either add or rebuild will result in extra entries to the boot menu, so
make sure you use names you will recognise when you see them I personally
add the drive and partition number to the description 0-1, 1-1 etc.)
Set the timeouts to your preferences.
Clean up the boot.ini from system properties in XP