Unmountable boot volume error is one of the most common bluescreens. It is usually caused by either a bad hard drive or corruption on the drive. If the drive is not bad and is just corruption the following guide should help fix the issue. If the bluescreen still comes up after going through all the steps below, it is most likely a failing hard drive. I will also give some tips on what to use to clone the bad drive to a new drive in this guide.
The first step would be to make insert the operating system cd or dvd and boot the computer from it. Make sure the computer is set to boot off the cd first in the bios.
If it is windows xp hit R at the menu to get into the recovery console. Type chkdsk /r. Check disk will then scan for errors and correct them if possible. Reboot the computer when it is finished.
If you use windows 7 you would choose repair my computer from the first menu and then pick command prompt. Then you would type chkdsk c: /r . It may need to run the check disk on restart. Hopefully after you restart it will run check disk and boot up ok.
If you still get the bluescreen after rebooting, then the hard drive is probably bad and will need to be cloned to another drive if possible.
You will need some software to clone the drive to a new drive. The most popular programs are Norton Ghost or Acronis True Image. There are a few more options out there but I would recommend one of those two. They are pretty straight forward to use. You might have to alter the settings on them to do a sector copy if the drive has a lot of bad sectors. In norton ghost you need to click on the (image all) option. I would also recommend running check disk on the clone after its done also to fix any corruption that might have copied over.
In the rare case the drive will not clone the only option is to install a new drive and reload the operating system from scratch.
Hopefully this guide will be successful in helping you resolve your unmountable boot volume bluescreen.