When using more than one hard drive on a computer, you must designate one drive a master and the other a slave before you install them. Your PC uses the terms "master" and "slave" to tell apart two drives on the same IDE channel. In most cases the master is the hard drive your computer will boot from. The term "slave" refers to any secondary device connected to the same IDE channel.

Before installing your new hard drive, locate the diagram for changing the jumper setting to a slave. The diagram depicts different sets of pins with a black box around them. First look on the top of your drive. If it's not there, see if your drive came with printed instructions or check the manufacturer's website.
To change the jumper, you may need to have a set of tweezers handy. Use them to reposition the jumper between the four male prongs and the IDE ribbon connector. Match the settings depicted in the diagram on top of the hard drive to make sure you do this correctly.
You may want to make sure your original hard drive jumper is set to a master. Chances are it already is since most hard drives come this way. You should double-check just to be sure.