So i had a system do this to me the other night. After removing a virus and rebooting several times the system appeared to be in working order. I shut down the system for the night. The system did not ever fully shutdown and in the morning it was "Forced off". After trying to boot again i got the BSOD with "UNMOUNTABLE_BOOT_VOLUME". This is a *mostly* patched windows XP professional machine. Mounting the drive in Linux doesn't show any apparent drive failures or data corruption.
A little digging turned up that if you boot off a XP install cd you can go into the recovery console.
First try running the following commands. Make sure you have backups first though because there is a chance of data loss!
1. chkdsk c: /r
Reboot when completed. If that doesn't do it reboot into the recovery console again and try this.
If your like me that still didnt fix the problem.
Reboot into bios (press del or f2 on system start) and look for the hard drive settings. I found the desktop i was working on has a SATA hard drive.
Under the drive controller settings there is an option to set the mode of the SATA drives.
Look for RAID AUTO ATA.
Make sure your drives are set to that and reboot. You should now load into windows again! If not you probably have a different error message and will have to go from there.