Alright for the purpose of this im going to assume your drive that is already in there is a master, so set your new drive to be a slave and put it on the second connector on your IDE cable and then just restart windows and it should pick up on it also remember to give the drive power with the P4 connector I believe it is and everything should go smoothly.
If your having problems make sure your jumpers are in the right area, to help you better I need a little more to go off of how your drive is already set up what kind of drive your putting in IDE or SATA and your OS.
If its SATA should be just as easy as plug and play, IDE should be almost just as simple as long as you get your drivers right.
http://compreviews.about.com/od/tuto...s/DIYCDDVD.htm
That tutorial should help you