XP - Unmountable Boot Volume
boot from the reinstallation disc
open recovery console
At the command prompt, type chkdsk /r , and then press ENTER.
At the command prompt, type exit , and then press ENTER to restart your computer.
This takes a bit longer, but the system should boot back into Windows.
If the above process fails to restore the system
repeat the steps by opening recovery console
then use fixboot command instead of the chkdsk /r command.