Installing a second HD is easy. I'm assuming you have a CDROM drive too.
First, unplug the computer. Then remove the case, and make sure you touch some metal like the power supply, to ground yourself and not pass on ESD charges to system components.
There are jumpers on the back of the HD and a diagram that shows you different settings for the jumpers.
First note where the IDE ribbon cable is plugged into the motherboard and the direction that the red line that runs down the cable is facing. That denotes pin one. Next remove the HD that you currently have installed. Set those jumpers to Master. On the drive that you want to install, set those jumpers to slave. Next, plug the HD into the middle of the ribbon cable. Make sure that pin 1 is facing the same direction on both HD's.
After you power your computer back on, go into CMOS and make sure that it is able to locate both drives and you're good to go. Format if necessary.
NOTE: If you have a hard time getting your ribbon cable to reach the location of the new drive, do the same thing with the jumpers for the CDROM and the new HD. Make the HD master and CDROM a slave and the rest is the same.
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Installing a second harddrive