When you say you "open the folders" are you sure you are including everything? How are you determining the folders to "open"? For example, are you using WIndows Explorer and then right clicking on all the top level folders and selecting Properties? In which case are you also taking account of the files that are in the drive's root (i.e not in a folder, but directly in C:) - this will include your page file and hibernation file plus maybe various other not so small items.
Also, what about the recycle bin? Have you emptied it?
Also you could try running CCleaner (click here)