My C drive is 20 GB and it shows I'm using almost19 GB But in windows explorer on my C drive Document & Settings, Program Files and Windows only add up About 13 GB (that 's all on my C drive). Showing my hidden files does not reveal anything. Wear are those gigs is my question.

If the computer was purchased from Dell, Gateway, HP or somewhere else similar, it may be that the hidden space is used as a recovery partition for when there are problems with your computer. Now days OEM computers do not ship with system recovery discs. Instead, they put everything into a hidden partition.

If you need another answer, please provide more information. Make and Model would help.