Why does boot volume unmounts.

It could be an indication of a hard disk that's about to fail. Go into My Computer then right click the C:\ drive and go to Properties. From the Tools tab, select Check Now and tick both boxes to find and fix errors automatically. Hit Apply and Ok and you should see a message that tells you it can't do the job now but will do on the next restart. Restart and let it go through that five stage process.

Are you sure it's not "Unmountable_Boot_Volume"? If so, you need to boot from a Windows XP CD, choose R at the first prompt and run chkdsk until there are no errors. It could also be a failing hard drive.