Firstly I ask have you ever opened the case of your PC before and know what you are looking at or for (I'm not trying to insult your intelligence by the way just trying to assertain a baseline of Tech knowledge)?
Assuming you know what all the stuff inside is, which I expect you do, do you have an extra 5¼" slot available to fit anoth HDD in?
Do you have spare capacity to connect the HDD to the Motherboard? i.e. is there an empty IDE connection to connect a ribbon lead to or a least a free socket on a lead that is already there?
Answer these Q's and we'll go from there if that's ok with you!!