Windows XP Pro Boot Error UNMOUNTABLE_BOOT_VOLUME

My windows XP Professional will not boot up in any mode – I have tried all the safe modes, normal mode and last good config mode it just reboots over and over. I disable the auto reboot and go the blue screen showing UNMOUNTABLE_BOOT_VOLUME STOP: 0x000000ED (0x823DCA20, 0x0000006, 0x00000000, 0x00000000. I do not have a windows start up cd. I am not sure what caused this,it is my sons computer, possibly some program he downloaded? Any way to fix it? Thank you…