I have this issue as well on both Windows 8 and 8.1. It is very annoying. I've ran memtest86+ thinking it had something to do with my ram, but it came back with no errors. I can't figure out how to stop my computer from randomly logging me out. I could be doing anything from browsing the web with a few tabs open or playing a game. Doesn't matter what I'm doing, it will happen. Thought it could be an issue from overclocking but the problem still comes back at stock speeds (cpu and gpu).
Specs if anyone wants to compare:
i5-3570k
sapphire vapor-x 7970 ghz ed
8gb ddr3-1600 g.skill
240gb g.skill phoenix III ssd
1.5tb WD hdd
700w antec psu
asrock z77 extreme4
I have also an asrock motherboard (z77 pro), all the rest is different. However with 8.1 my issues are solved (90% certain). I have been running for 3 weeks now without any problems. I opened a lot of stuff and no problems at all (22 GB ram in use atm).