OK, I've double-checked it and there's no malware.
All I can think of is that something got messed up from when I upgraded the motherboard. I did image the OS partition and restore it using Universal Restore, which is meant to reset the devices/drivers but I still found some AMD devices from the old motherboard lingering in Device Manager. Although I've deleted those manually now, maybe there's something still lingering and causing problems, so I'll have to try a fresh install of Win 8.1 sometime.
I've actually put the other 8GB back in now and disabled the pagefile, so it's less of an issue but I still get the low memory warning if I launch ArmA3 with Chrome with all my tabs open. If I close Chrome first it's fine though. It could just be that Chrome is a memory hog, as currently it's showing about 6GB Memory, 7GB Virtual Memory (Private) and 2.5GB Virtual Memory (Mapped) and Task Manager shows 7.9GB In Use, 7.6GB Available and 14.0/15.9GB Committed, with 8.1GB Cached. It seems the problems arise when the Committed gets near to full, regardless of how much Available is left, so I don't know what that's all about.