My Windows 8.1 computer wakes up immediately after being put to sleep (either of the sleep options in the start menu.) So I spent some time trying to understand the problem, read multiple posts here with similar issues, but got nowhere, so posting this.
The weirdest phenomenon is that when I click "turn off" from the start menu (loosely translated as the Windows language is not English), the computer shuts down, but then immediately turns itself back on! Likewise when I turn the PC off using the hardware power-button, it shuts downs (I get a normal blue screen saying "windows is powering off") but then immediately the PC powers back up.
I ran "powercfg -lastwake", and found no clues. When powering up from software or hardware power-down, its says:
Wake History Count - 0
When powering up from sleep it says:
Wake History Count - 1
Wake History 
Wake Source Count - 0
What does that mean?
In device manager I unticked "allow this device to wakeup computer" for the mouse and keyboard (just to rule this out, as eventually I do want these to wake up the computer). I unticked wakeup option for the LAN card (unticked magic packet too).
I tried to look at Event Viewer, I found a Power-Troubleshooter event from the time I sent the computer to sleep, that says the system went to sleep for about 1 minute, wakeup source unknown. I'm sure I haven't exhausted Event Viewer possibilities, what should I filter for in Event Viewer?
I checked task manager, there are no tasks. I think task wakeup isn't likely since self- power-up happens immediately each time regardless If I do sleep or power-down.
BIOS of this PC is far from being the most updated version, do you recommend updating the BIOS? The computer is rather ancient, Intel DP35DP, the BIOS documentation recommends not to update unless your particular issue appears in their updates/fixes lists, to avoid new issues. So that made me a bit reluctant to update.
Another thing: in control panel, power options, advanced, under the options for "sleep", I set "wakeup timers" to "not available" (loosely translated).
Any help will be greatly appreciated.