Hello Jack--
I'm betting you have a very viable alternative to installing Win XP on C:\
from scratch. You did not tell us why you think you need to reinstall it,
but let's assume it's systemically trashed or has something you deem is
keeping you from using it properly--it could even be one component that
irritates you--I am only guessing because you didn't specify why.
However, I suggest that before you resort to formatting and reinstalling XP
you get your XP back to normal by doing
a repair install. We're talking a few minutes to set it up and then 32
minutes of the XP setup. That way you don't lose anything. The requirement
for this is an XP CD, which you must have if you were planning to format
and reinstall it.
If that doesn't work, and you format and reinstall XP, then if the dual boot
hasn't survived you can try putting in the Vista DVD and using either
Startup Repair from the Recovery link as Dustin suggests, or another
alternative if that doesn't work is to try a system restore from the same
link.
Repair Install XP
http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/tips/doug92.mspx
How to perform an in-place upgrade (reinstallation) of Windows XP
http://support.microsoft.com/kb/315341 (Method 2 Booting from the XP CD)
You could also try F8 advanced options (4 Safe Modes--->system restore) and
LKG for Vista if nothing else worked first.
CH