Missing storage?

So I've recently installed bootcamp for the 3rd time on my Late 2013 MBP and I've allocated about 60gb of my 1TB SSD for Windows 8.1 Pro 64-bit. When I open up "My Computer" it tells me that I have 4gb free out of 60gb, but all my files added up to only 36gb (+4gb is 40gb out of the allocated 60gb), which means there is 20gb of files hidden somewhere in the drive. Does anyone know what I can do to free up that 20gb of space?