Whenever your PC wakes up, Windows keeps a log of why. To view, open a command line and run:
C:\> powercfg -lastwake Wake History Count - 1 Wake History  Wake Source Count - 1 Wake Source  Type: Wake Timer Owner: [PROCESS] \Device\HarddiskVolume3\Windows\System32\services.exe Owner Supplied Reason: Windows will execute '\Microsoft\Windows\Media Center\mcupdate_scheduled' scheduled task that requested waking the computer.
As you can see, my computer woke up because of a scheduled task. Open the Task Scheduler: And navigate to the task in question (\Microsoft\Windows\Media Center\mcupdate_scheduled, in my case). On the Conditions tab, uncheck wake: