First off, I will assume that you mean the hard drive light is always on and its always making some kind of clicking noises. I assume this because while the computer is on, the drive is constantly spinning anyway. If the drive is always accessing something, clicking away, then yes, its getting more wear and tear than if it was just sitting there spinning idle. There are several possible reasons for this.
One is that your computer doesn't have enough memory and therefore is using the hard drive as memory. You could try adding more memory to the computer in that case.
Another possibility is that you have a virus or lots of spyware on your computer, or you are running lots of other programs that aren't necessary, and this is consuming memory and/or accessing the hard drive all the time. You can try running antivirus software like AVAST! (home edition is free) and antispyware software like Ad-Aware (home edition also free). These would eliminate most problematic programs.
Generally most people have a lack of memory in their computer, if you are running Windows XP with 256mb of memory you should definitely upgrade to 512mb minimum.