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?
Many thanks guys for your many answers;; The figures are approx,,24GIG used space and 14 gig free space on the disc. Folder content accounts for about 15 gig of the used space therefore I can't account for about 9 gig of the 24gig used space. Reducing system restore gained me about 1gig.. I apologise that my original approx figures led to ambiguous equations. Will try c cleaner later..thanks again