As rabid said, there are many hidden folders, and individual folder sizes are not 100% accurate.
There as many reasons your HDD space may fill up quickly. again as rabid said, a log file that is not limited in size. I had an SQL log file on of my servers that grew out of control and crashed my server a couple times before I caught it.
One way to find it is do a file search by modified date, and leave the search criteria blank. This will list the most recent files to see a change then confirm what they are and why they may have changed, and in particular if one is very large.
Another cause that I haven't seen in a while but have in the past is a replicating virus. It essentially just keeps making copies of files on your drive to fill it up and cause it to crash. This usually happens pretty quick though, like over the a couple hours.
Also, check your temp folder restrictions, and your browser history purge durations, as they can help keep your system from filling up with unnecessary information.