ok, so you have somewhere to mount it at least, the cable that connect your current hard disk to your motherboard, does it have any other items connected?
the cable that connectd the hard disk, CD-ROM ect, is an IDE cable, its the fat ribbon one. unless you got the rounded ones
so unless it's got one connector on the ribbon, you should be able to put another hard drive. Trace the hard drive cable and check for another connector like what is plugged into your current hard drive.
i just tracked the IDE cable to where its connected to the motherboard. all of the IDE slots on the motherboard are taken up. would it still be possible to add hd?