Unmountable boot volume errors are usually overcome by running the chkdsk /r command (note space between k and /).
This command is available from within Windows and from the XP Recovery Console.
Users who don't have an XP CD...can download the RC in .iso file format...and then burn that .iso file to CD, producing a bootable CD that will provide access to RC commands.
Download Recovery Console, Bootable ISO - http://www.webtree.ca/windowsxp/repair_xp.htm
The .iso file can be easily burned to CD by using any one of the following programs/techniques: How to successfully burn or write an ISO-image to cd or dvd - http://iso.snoekonline.com/iso.htm
IMO, the Active@ Iso Burner program is easy to use for such.
Once the bootable CD is burned, place it in the optical drive.
Reboot the system, go into the BIOS/Setup...and change the boot order so that the optical drive is the first option. Save the changes by hitting F10, which reboots the system and then the system will try to boot from the CD. Some screens will reflect a "Hit any key...." message to boot from anything other than the hard drive, so pay attention to the screen.
How to use CHKDSK in the XP Recovery Console - http://pirules3.14.googlepages.com/recovery_console_chkdsk