I haven't tried it because I don't need to do it but VISTABOOTPro that
I installed before installing VISTA so I wouldn't have to struggle with
BCEDIT to change default boot etc in the boot up menu apparently has a
command that I presume does the necessary change to boot files for you
and then allows you just to delete the unwanted system.
Check out their website since it would be nice if it did the repair and
left the system ready just to be a single OS one.
http://www.vistabootpro.org/
Here's a thread in their forum about the specific topic of removing XP
from a dual boot with VISTA:
http://www.pro-networks.org/forum/about90817.html
but I think there's definitely something in VISTABOOTPro for removing
an OS.