A friend brought round her computer today that was constantly rebooting while Windows was trying to boot. After a bit of fiddling I got it so that I could see the error message which was a BSOD (blue screen of death) with the message “Unmountable_Boot_Volume” on it.

A littile googling found this post on MSGoodies with the following details:

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. You can easily mix languages – I used an English CD on a Danish Windows

Run these command –
chkdsk c: /r /p
fixmbr
exit (to reboot)

And it worked great, saved me alot of fiddling. I’ve put the details up here in case I need it again in the future.