Why is it so big though?
On my system the Windows directory is only 5GB.
I know in the past, any Windows updates would be cached in your windows directory forever and there was no safe way to remove them. That would gradually grow the windows directory size forever. Also, it seems there are a lot of files that get thrown in there when you start installing multiple versions of SQL Server or Visual Studio.
Disk Clean-up.
Clean-up System Files.
Windows Update Clean-up (and Temporary Windows installation files).
Ok. Ok.
Enjoy.
I assumed it WinSxS related as the other reply mentions, and upon investigation found that it is 9GB alone! I don't run Windows outside of work so I've just never really bothered looking into it, but if it continues to balloon I guess I'll have to.