Denise,
Many things can cause a computer to make a clicking noise. It is NOT necessarily your hard drive. You could have something in a fan, a wonky cd drive, or a few other things. Best to make sure before you go through all the trouble of a new hard drive.
Go to your Control Panel, into Administrative Tools, into Event Viewer, then click on the "System" entry on the list on the left side. Do you see a lot of errors (X in a red circle) with a "Source" of Disk? If you do, it's time to back up and get a new HDD.
If you don't, look around that log a bit and tell me what you do see in the way of errors. There are many things that will show up as errors that are not particularly worrisome, and a few that are. Just give me a general idea of the amount of particular errors (not Warnings or Information entries) and I can help more.
Don't worry, I'm a professional. I do this every day, all day.