it it's only three years old I'm assuming it's in one of those funky black (midnight grey) cases, with the little green things to secure it in place?
is it a tower case, or a desktop case? if its a small form factor desktop case then there might simply be no room for a second hard drive.
normally, there may be some space underneath the floppy drive, where you can install a harddrive. or there may be some space above the old drive....
if it has the little green things then you should unattache some of the space ones from the case where they are stored, attached them to the drive and then push it into position, (till you hear the cilps snap into place) in an avai;able space, (before you use the clips thies space will have seemed too big).
if there are no clips then there should (hopefully) be a spare space in a harddisk cage somewhere, (as I said this might be under or on top of the existing hard drive, or the floppy drive. or in some cases, there is a space under neath the larger cage where the CD drives are bolted in, (useually the CD drive cage comes out if thise is true).