Diagnose the 'Unmountable Boot Volume' Error
1. Determine if you have your BIOS settings configured so as to force faster UDMA modes. In this case, you can solve the problem if you restart your computer, enter the BIOS and then load the 'fail-safe' default settings. Next, reboot the computer and the problem will be fixed.
2. Find out if you used the wrong IDE cables. This error can commonly be caused when you swap hard drives and use the wrong IDE cables to connect them. You may experience this error if your computer uses Ultra Direct Memory Access, or the UDMA hard-disk controller, with a standard 40-wire connector cable. In this case, you can fix the problem simply by using the correct IDE cables when using these hard-disk controllers.

3. If none of these possible causes corresponds with your problem, then complete the steps in Section 2.
Resolve the 'Unmountable Boot Volume' Error
4. Boot up your computer from your Windows XP installation CD. Insert the XP CD into your CD-ROM drive and turn on your computer.
5. Set your BIOS to boot from CD. Your system’s BIOS settings can be opened by hitting the F2, F8 or Delete key on your keyboard as the computer boots up. The specific key differs depending on your computer’s manufacturer. When you see the message 'Press any key to boot from CD,' tap any key on your keyboard and wait until you see the “Welcome to Setup” screen.
6. Hit 'R' to use your Recovery Console to repair the installation. If you have a dual-boot computer, you need to select the particular Windows installation you would like to access.
7. Type in your administrative password when the computer prompts you. If you have no password, simply press Enter. This will bring you to the command prompt.
8. Type 'CHKDSK /R' and press Enter. CHKDSK will now scan and repair your hard drive. Type 'Exit' when this process is complete
9. Press 'Enter' to restart the computer. Your computer should now boot properly, without that annoying error.