C Drive nearly filled

I am running Windows 7 Pro for video editing.
Intel Core i7-3930 k CPU @ 3.20 gHz
64 Gb RAM
111 GB C drive with 12.8 Gb free; 990 Gb D drive with 860 Gb free.

My issue is with the quickly diminishing free space on the C drive.

I have only 7 folders on C Drive:

INTEL 840 KB, 3 files in1 folder
PERFLOGS 15.2 MB, 39 files in 4 folders
PROGFILES 1.49 Gb, 3,004 files in 284 folders
PROGFILES (x86) 1.2 Gb, with 7,157 files in 1,681 folders
PROGDATA 768 MB, 6,228 files in 543 folders
USERS 47 Gb, 9,562 files in 837 folders
WINDOWS 17 Gb, 79,000 files in 15,926 folders

It seems that I should have a lot more than 12.8 Gb free on the 111 Gb C drive

I have removed several programs, defragmented the drive, cleaned up the drive; but only regained 1.6 Gb.

The C Drive User folder shows only 4 folders:

Default 1.36 MB
Public 165 MB
UpDatusUser 17.7 MB
User 46.8 GB

I have no idea where to find all these folders or which ones I can remove. Any help would be appreciated. I am wanting to install Avid Media Composer 6.5 and there is simply not enough room.