uninstall a reasonably sized game and you should have no trouble. No computer should have less than 80 GB in it...well i wouldn't even call that reasonable these days...go to a local computer store and pick one up...
I'm not trying to be patronizing or condesending but i do get the impression by your question that you would not be comfortable installing a hard drive yourself. I'm also assuming you're speaking of a desktop computer and not a laptop...I would have the computer store install the hard drive for you also.
Anyway, i guess the two posters above me have earned your thanks for their fine attempts at helping you with your problem.
Hope they get the same responses when they ask if anyone knows of a good mechanic or help with something not computer related that they don't know anything about.