Very easy to do. If you open up your PC and locate the existing hard drive, either directly above or below it there will be an empty cradle that will take the second drive. This is the place where you may encounter a problem or two. Frequently it is necessary to remove this cradle altogether to get at the screw fittings on both sides, but if you logically study the layout it should be possible to sort it out. On the rear of the new drive there will be a very small jumper linking two pins. Following the printed guide on the top of the drive, set this jumper to either Slave or Cable Select. They are a bit fiddly to remove and handle, be careful not to drop it. Then fit the drive into the cradle with two screws either side, and replace the cradle in its original place.
Locate the two cables you will need. One four pin power lead coming from the power supply, and one multi-pin IDE connector (coloured grey) on the end of the ribbon cable. Connect these to the new drive making sure they are firmly pushed in as far as possible (they will only fit one way up)and that's it. Hopefully it will be detected when you boot into Windows.