The only way to find out is to open up the machine and see. Depending on the chassis, you may have room from 0 to 10 hard drives in there. Yes, as a final resort you can throw out the floppy drive and use that 3.5" bay to hold the hard drive.
It should be easy to open (maybe not bvious at first glance, but easy) - some srews in the back or some latches somewhere. If you are not technically inclined enough to open up the computer then you shouldn't think about messing with the contents at all..
We are talking about a desktop computer here, right? You can't add hard drives to laptops - only replace the existing one.