Disk Utility Shows less free space than there should be

I recently had to reformat my hard drive and reinstall the os at which time the system profiler showed free space of approx 35 gigs (80 gig hard drive) after I reinstalled my programs and files. I have downloaded very little to my drive since and now it shows approx 7 gigs free. When I look at the file size of the different folders, it adds up to approx 38 gigs,,,does this mean that there is something wrong with the hard drive? Is it fixing to crash? I ran the disk utility and mac janitor programs and that didn't seem to make a difference. Any light you can shed would be greatly appreciated. Thanks.