Depends on how old, and what interface technology it uses. It's probably IDE, but not necessarily.
Here is a picture of an IDE connector:
http://static.howstuffworks.com/gif/ide-ribbon.jpg
And here are some SCSI connectors:
http://www.scsita.org/aboutscsi/connectorGuide.html
Now here is a picture of a SATA connector:
http://www.antec.com/images/sata_connector.jpg
So, assuming it isn't an SCSI drive, then it is an IDE drive, as there probably weren't any 15 gigabyte SATA drives made. The next question is what is the existing drive in your system? If it is SATA then it has a SATA connector and a thin cable leading down to the motherboard. If it has a wide, flat cable (ribbon cable) then it is an IDE drive. Of course there are thinner IDE cables too, but they are mostly used by enthusiasts. Check the connectors if there is any doubt.
If it is an IDE drive, then the cable probably has a second connector attached that isn't being used. IDE drives have to be identified as either master or slave. The existing drive is definitely the master already, or it wouldn't work. Most modern IDE drives are cable select, that is they know they are master or slave by which connector they are plugged into. I think the one on the end is always master, and the middle one is slave. However, older IDE drives were not cable select. They have a jumper on the back near the connector that determines master/slave status. You probably will have a jumper. If you're lucky the drive will have a sticker on it giving the jumper pin assignments, and telling you how to jumper for master/slave. If not check the OEM website. Jumper the drive for slave, put it in the machine, attach a four-pin molex connector from the power supply to the drive for power, and then plug the middle IDE connector into the back. Power back up and the system should detect the drive automatically, allowing Windows to mount it.
If your current drive is a SATA drive, then you will need to purchase an IDE cable, and jumper the drive as master. Plug the cable into the IDE connector on your motherboard, install the drive and attach power, and plug the far end of the IDE cable into the drive. Now power-up and the system should see the drive.