Take a look at Unmountable Boot Volume - http://support.microsoft.com/kb/555302
The normal fix for this is to run the chkdsk /r command (or equivalent).
To run the equivalent:
Go to Explorer view, right-click the partition/drive in question...select Properties.
Click on Tools tab...push Check Now button (Error checking).
Select both options (automatically fix and scan for) and click Start/Enter.
A screen will open and inform you that chkdsk cannot be run now, etc...type Y and hit Enter.
Reboot the system, command will execute. The system will boot into XP when command has completed.
Edited by hamluis, 25 September 2009 - 05:45 AM.