Windows lost access to the system partition or boot volume during the startup process. Typical causes: Installing incorrect device drivers when installing or upgrading storage adapter hardware, or a virus.
OR it could be this( from PCHell website)
Causes of Unmountable Boot Volume
Many times this error occurs when I have swapped a hard drive and used the wrong IDE cable to connect it. If your computer uses an Ultra Direct Memory Access (UDMA) hard disk controller, and you use a standard 40-wire connector cable to connect the UDMA drive, you may experience this error. Make sure you are using the correct IDE cable.
Also, if your BIOS settings are configured to force the faster UDMA modes, this error may occur. In this situation, restart your computer and enter the BIOS and load the "fail-safe" default settings and reboot.
If neither of these issues are the cause for the Unmountable Boot Volume, then the issue most likely is caused by a damaged BOOT.INI file in the root directory of the boot drive or file system corruption.
Follow the steps below to correct these file system issues:
1) Start your computer with your Windows XP CD-ROM, or with Windows XP boot disks
2) When the Welcome to Setup screen appears, press R to Repair the installation using the Recovery Console
3) If you have a dual-boot or multiple-boot computer, select the Windows installation you want to access
4) Type the administrator password when you are prompted, if no administrator password is set then just press Enter
5) At the command prompt
, type CHKDSK /R and then press Enter
6) Once CHKDSK has finished checking and repairing the hard drive, type EXIT and press Enter to restart your computer
If this procedure does not work, repeat it and use the fixboot command in step 5 instead of the chkdsk /r command. FIXBOOT writes a new partition boot sector to the system partition. The fixboot command is only available when you are using the Recovery Console.
For more information on FIXBOOT, visit the Microsoft Documentation page for FIXBOOT.