There should be a space need the old drive to put it in. Also on the Hard drive ribbon that is already fitted there most likely is another plug this should go to your new drive Plus the new drive should be set to Slave there will be a jumper on the back edge of the drive keep your fingers of the printed board, on the ribbon cable you will see that one side of it as a red wire or tracer this should go next to the power socket that goes right next to it this only goes in one way. start your comp and see if it recognises the new drive if it do not, you will have to go into cmos when the computer is starting, look for a line something like this " To enter Setup Press Del" if you can get into cmos the instructions on how to navigate are normally bottom right as you cannot use your mouse got to the page to auto detect the drive. When you have done that exit and save settings. When you get back into windows go to Windows Explorer if you see the new drive right click on it and choose format. it will then be ready to use
If both your IDE primary and secondary already have two devices fitted on each channel then do as the suggestion above says. Purchase a Promise PCI ATA133 IDE controller card. This will allow you to fit two more devices. If you have a tower case you may eventually use both of these.