As Thinkgood says, the hard drive is probably corrupted (no longer bootable).
Something to try: If you can come up with a win95 or win98 boot disk with fdisk on it (I assume this oldie has win95 or win98), use it to boot up to dos. At prompt type "Fdisk /MBR" (without the quotes) and hit enter. This will rebuild the Master Boot Record. That may be all it takes - all data still there. If that doesn't do the trick you may have to reformat the c: drive and reinstall windows.
This can happen when "trying to make it go faster". The kids may have increased the speed of the Front Side Bus to overclock the processor a bit. Usually there is no harm down - it either works or it doesn't and if it doesn't you just set the FSB speed back to its default setting (50 to 66 Mhz for old pentiums). But, when you increase the front side bus speed, you usually are also increasing the PCI bus speed and the hard drive is controlled via a PCI IDE interface. Some hard drives are not tolerant of an out-of-spec PCI bus and will become corrupted (but not usually damaged).
It could also be that they selected incorrect hard drive parameters. You could look through the bios screens (hold Delete during boot-up) to see if there is a HDD autodetect option.
