I don't know Windows well enough to fix an existing installation, but you could always boot up into a live Linux environment (Knoppix is designed for this, but you could just use the install CD of almost any distro), backup the data to external storage devices, erase the hard drive, and reinstall Windows from scratch. If they lost their Windows disk and if you don't have a spare one, maybe now would be a good time to get them off Windows forever...