Used hard drive space?

I have a Western Digital 160gig HD. My hard drive reports that 61.2gigs are currently in use… When I open up the C Drive and select all the directories below that(with all hidden and system files selected). It says that I am only using 42gigs… I also used a 3rd party utility that shows that is the total amount being used.

I do not believe that my HD is failing… I ran chdsk… The drive seems fine…