I have a laptop which today showed this problem when I turn it on. The day before that I was partitioning my external HDD in that laptop using Management Disk in XP but I thought that wasn't the cause since I was only making changes in the external HDD. Did some googling and the fix was to use the XP recovery disk, but my laptop is a netbook and doesn't have a CDROM. Is there a way to fix this problem?
More aboutlaptop unmountable boot volume
If you have a built-in recovery partition, you might have the option to do a Windows Repair.