|Make sure that you are also looking at hidden files and folders as well as system files when calculating space. |
In folder options, click on the radio button for show hidden files and folders, also be sure to UN-CHECK the box for "Hide protected Operating System Files". This will allow ALL files to be visible when looking at various folders.
Alternatively, you could also download a free program called WinDirStat which will show you, in both tree and graphical forms, what space all of the files and folders are taking up on your drive. It can be found for free directly from the developer here: http://windirstat.info/. You could also get it from any of the major reputable free software download sites (eg. cnet/download.com, majorgeeks.com, etc)
Let me know if you have more questions.
IT Desktop & Network Consultant - MOS Master Certified, MCP, MCITP - Windows 7, CCNA Certificate Pending, A+, Network +