Try this, Tools, Folder Options..., view, now un-tick 'Hide protected operating system files(Recommended).
Open the C: drive and you should see some folders lighter than the rest and you've not seen before. ie System Volume Information.
Drag over these revealed folders and right click, properties.
You will probably now account for the missing space, these folder are for system restore and some other stuff like all the porn sites you've ever visited.
This will account why it's so large. lol