It is indeed rather worrying that it only sees an optical drive (I assume you don't have a SATA hard drive, considering the apparant age of your system).
If you have the optical drive and the hard disk on different IDE cables (which I suspect you don't), try swapping the cables between the two units. If the disk turns up and the optical unit vanishes, you have a bad cable. Remember to get an 80 pin cable.
If they're on the same cable, disconnect both the optical drive and the HDD and try using the connector that was used for the optical with the disc. If you get the disk back this way, get a new cable.
You might also have a confused BIOS. To fix this, you need to clear the CMOS memory. It varies between different motherboard manufacturers how this is done, if you can't find the procedure for yours in the manual or on the manufacturers website, have a look at the motherboard to see if you can find two pins with "CMOS", "CMOS clear" or something similar printed next to it. If it has a jumper, remove it. If it doesn't have one, put one in (you should be able to borrow one from the optical drive if you've already disconnected it). Some motherboards require you to power cycle the motherboard with the jumper added or removed.