(I have ESET NOD32 Antivirus but I noticed this current thread and describes my problems almost identically.)
I've been running ESET Anti Virus v7.x on Windows 8.1 Pro on my desktop for about a year and a half. (I had previously tried updating to v8.x, but it blocked some of my CableCARD tv tuner functions from working in WMC.) This is a custom built computer and has always been ROCK SOLID. Starting this evening, the system completely locks up with no BSoD or memory dump. This seems to happen between appox 3 and 10 minutes after logging into Windows. After this occurred a half dozen times, I tried disabling ESET's protection until next restart. (I got the red triangle warning in the system tray.) A few minutes later, I noticed the ESET system tray icon doing the blue circle spinning like it was trying to update and the system almost immediately froze. After restarting Windows, I uninstalled ESET and everything was fine. I did multiple restarts, used a number of apps, and it all appeared fine.
So I downloaded the latest version of ESET v8.x from ESET, installed it, and restarted my PC. Sure enough, after about 5 minutes, the computer froze. So now it's happened with both v7.x and v8.x. It doesn't happen when ESET is uninstalled.