you can certainly fit a second drive.
1. Make sure that the jumper on the back of the drive is set to the slave position.
2. Connect the drive to the same ribbon cable as the master drive - the primary IDE cable.
3. Make sure you've connected a power lead to the new drive.
Boot the computer and watch for the drive to be recognised in the BIOS as the startup routine runs. When Windows has finished loading, check that a drive letter has been assigned to your new drive. If this doesn't happen you may need to reboot and enter the BIOS (press the delete key as the computer starts) to get the drive recognised. If you need help with that aspect of things come back to this thread.