I get this message when I turn on my computer. First, a black and white screen asks me what mode I want to start in (ie. safe mode etc.), then I click on any mode and a screen with 'UNMOUNTABLE_BOOT_VOLUME' appears.
Start the recovery console. If you do not have it installed - or when that version does not work - like in this case, boot from an installation CD and select R for repair.
Run these command -
chkdsk c: /r /p
fixmbr
exit (to reboot)