Conveniently, my screenshotting tool logs running foreground applications, so I can tell you exactly what I had running at the time that shot was taken:
iexplore
explorer
mirc
Photoshop
MMix_64
mirc is not actually installed (running from a portable drive) and I had Photoshop running only briefly after having just installed it. I believe the issue was occurring before that was installed, but I cannot say that with absolute certainty. I don't remember what I had open in IE, but it's a safe bet that at the very least I had shotweb and this forum open.
As for background processes and the file signing time and whatnot, I can provide that next time I'm there.
I can tell you, however, that I am reasonably certain the MMix on that system is a fresh non-upgrade install of 1.11, as I reformatted the system yesterday, and chose to wait until the next MMix release (which wound up being 1.11). 1.10 is the only other version I've used with any Win8.1 install, and that version had different issues which this install is not exhibiting. Also, my downloads page currently only provides 1.10 and 1.11, so I feel it's safe to say that this is 1.11.
So, the system is an almost totally-clean install of 8.1 RTM with little more than drivers and my development tools installed. Most of the applications listed below have only been ran once or twice, even.
Installed applications (from memory):
Visual Studio 2008 SP1
Visual Studio 2010 SP1
Visual Studio 2012
ReSharper 8
dotPeek
Unity3D 4.2.1
UnityVS 1.2
Photoshop CC
Office 2013 Professional Plus
MMix 1.11beta
CCCP
LatencyMon
7-Zip
NVIDIA GPU driver 327.xx (EVGA GTX 480)
Intel RAID manager/driver (EVGA P55LE motherboard)
Applications ran from a storage drive (not installed) include mIRC and uTorrent.
As a sidenote, I do have the Japanese input method enabled in addition to English (which is what makes that [A] box appear when I focus the textbox).