Whenever you get the Blue Screen of Death stating unmountable_boot_device this usually is a simple error to fix.

1. Boot off the Windows XP CD and go into the recovery console.
2. Enter chkdsk c: /r  (for example this will check the C drive if you only have one drive this is the default)
3. Enter chkdsk /p
4. fixboot c:

This should fix any errors and allow the system to boot again.