Not normally if some of the following is true.
If you installed Vista second without using a third party boot manager
and a separate system partition exclusive of the XP drive, your XP drive
is probably your system drive. Vista would normally install the
bootloader to the system drive(i.e. your xp drive)
Search this newsgroup for dual boot topics..a lot of info is present on
removing Vista as the second o/s in a dual boot, dual drive system.
Here's some additional reading info.
In almost all cases, folks usually suggest using a third party
app(BootItNg, VistaBootPro, EasyBCD) to remove an os from the boot
loader screen, or run a Vista repair install, or clean install Vista
and/or a combination of the these when one wishes to format the XP
drive.
...winston
MS-MVP Windows Live Mail