An Unmountable Boot Volume error occurs when your disk drive has errors on it that then mess up Windows XP. If you still have your XP installation disk, you can still recover from this error so that you can get the remaining good data off your drive.

- Turn off you computer, insert the install drive, and turn your computer back on.

- When you get to the welcome screen, press R on the keyboard to get into recovery mode.

- Once you get the DOS prompt, type chkdsk /p and hit Enter. This will start the utility that checks your disk. It'll take a while.

- Once the disk check is done, type fixboot and hit Enter. This will attempt to repair your boot disk. When you are prompted to fix errors, always select Yes by hitting Y.

- When done, type exit and hit Enter.

Reboot your computer and hopefully this will get rid of the Unmountable Boot Volume error. If your system is up and running, remember, your drive has problems. You should replace it as soon as possible and also back up all the data on it as soon as possible. While there is a chance the drive is still fine and just had "one time" issues, do you really want to trust your data to it at this point?