You didn't say if this was a mini-tower or laptop. I added a second HD to a tower a few years ago and it was a breeze. The power supply should have extra unused power plugs so pick one close to the HD your adding. Also there
will be a ribbon cable attached to the existing HD and it should have an unused connector, if not you may need to buy a new ribbon cable with extra inline connectors. Before starting you'll need to change the pinout on the back of the HD to make it the slave. It's a matter of moving a cover to expose the correct pinout for the slave setting.
There are lots of how-tos online here is just one.
http://www.helpwithpcs.com/upgrading...hard-drive.htm