The Windows Side-by-Side folder (C:\Windows\winsxs) keeps different versions of files so that programs only see the versions they expect. The problem is this folder grows every time you install an update or new program, and never shrinks when programs are uninstalled and the files are no longer needed. I just managed to clean 5GB out of this folder using the command from Microsoft described a the following website:
http://www.iishacks.com/2011/06/23/reduce-windows-7-winsxs-folder-size/
This was suggested a few years back, but at that time no one was aware of the microsoft command to clean the folder so no one wanted to go directly deleting things out of the Windows directory. Now that this Microsoft command is known, it seems like it would be a simple thing to add support for cleaning this folder and saving a lot of disk space. Thanks!