I recently updated my workstation from Windows 7 to Windows 8 Pro. After the upgrade, the computer seemed to wake up by itself from hibernation daily (I usually use hibernation instead of system shut down to preserve the state of all open applications).
I have set up all devices (specifically, the mouse, keyboard, and the network card) to be unable to wake up the computer in Device Manager, and have also configured the current power plan to Disable waking up the system on timed events (from the administrative advanced settings section). But apparently these were not enough.
Today I noticed that the waking happened around 4 PM (I was working on a notebook at the next desk). Armed with this information, Ioana has finally discovered that a task called Media Center Update was scheduled to run every day at 3:58 PM, and it had the Wake the computer to run this task checkbox set. Hopefully, unsetting the checkbox would eventually get the problem solved.
If you have the same issue and want to get to the scheduled task settings on your system, follow this path: Control Panel, System and Security, Administrative Tools, Task Scheduler (running as administrator), Task Scheduler Library (from the left tree-view), Microsoft, Windows, Media Center, mcupdate_scheduled, Properties (in the context menu available on right clicking).
Update: It seems that was it! One day after, the computer hasn’t waked up by itself again.