Solution:
Run this program-> Rammap
http://blogs.technet.com/b/askperf/archive/2010/08/13/introduction-to-the-new-sysinternals-tool-rammap.aspx
It's probably something like a Microsoft Search function. Task Manager doesn't report everything, though I think anything hidden would have to be part of Windows 8.
In my case I think I had to disable Microsoft Search, but find the program causing the issue then Google the name of it, mention high memory usage and find a solution.
johnbl,
Your above solution can't be correct as he's using 9GB of memory. Look at the picture he includes. There's no cap set anywhere. If the system had a memory cap, then it simply wouldn't use any more for anything.
scottirvine,
BTW, you are creating a bottleneck by running three sticks of RAM. That forces the system to run Single-Channel mode instead of Dual. Look at your motherboard manual, then use the configuration for TWO sticks only. I'm guessing you have 4+4+2, so just use 8GB (2x4GB) instead. It depends on your CPU and game/program you are using but you might be losing between 10 and 40% of your performance.
More info:
I have 8GB currently installed, and when I turn on my Windows 8.1 PC it uses about 2.4GB. It sometimes jumps up to 6GB or so if I game but usually drops back down below 4GB after the game. While Windows does cache things, the memory manager should never use as much as it has in your case. Again, mine was a memory leak with the Windows Search program or similar function. As I write this I'm at 2.4GB/7.9GB looking at the same graph where you show 9.0/9.7GB but I was over 90% before I discovered the problem which RAMMAP helped fix. I had 16GB when my large memory usage happened (waiting replacement memory so have 8GB now) and the Windows program was using over 10GB of that.
Windows 8.1 has some memory manager fixes as well.