Your computer will not boot returning the error "Unmountable boot Volume". This is usually caused by a corrupt "BOOT.INI" file.
SOLUTION 1
Access the recovery console. (This is the first repair option you get after you boot off the Windows XP CD (To see where this menu is
Click here), unfortunately some OEM CD's (Or recovery CD's) do not have this option), but if you download the Windows XP boot disks you will be able to use the Console.
Download XP boot disks from here (May require 6 blank floppy disks)
Once you log into the recovery console type the following command "CHKDSK /F" or "CHKDSK /P".
Another option to try, which was sent in from "Maria in Atlanta", which she found that worked was to run the "FIXBOOT" command. To do this simply type "FIXBOOT". Using the "FIXBOOT" command without any parameters will write a new partition boot sector to the system partition you are logged on to
Now reboot the computer. You should be able to log into Windows.