Geting rid of Windows XP after a Vista Basic Upgrade install

Because of limited drive C space Vista was installed to drive D and now I'm
stuck with an unwanted dual boot system with Windows XP on one partition and
Vista on the other. Because I have the upgrade version I suspect I have
limited options (e.g. I don't think I can boot from the Vista disk,
repartition and re-install Vista as windows must be running for the upgrade
version to install itself. How do I get rid of the XP part of the dual boot
and reclaim all the space on drive c (I don't need anything on drive c)?
Thamk you