One night I'm working at my desk with my computer off. Suddenly my computer turns itself on. I've disabled mouse and keyboard inputs from waking it up. The only thing that should wake it from sleep is when I press the power button. Luckily there is a command that can tell you what woke your computer up. Open a command prompt and type "powercfg -lastwake". Here's what I saw:
C:\Users\Dustin>powercfg -lastwake Wake History Count - 1 Wake History  Wake Source Count - 1 Wake Source  Type: Wake Timer Owner: [PROCESS] \Device\HarddiskVolume1\Windows\System32\services.exe Owner Supplied Reason: Windows will execute 'NT TASK\Microsoft\Windows\Media Center\mcupdate_scheduled' scheduled task that requested waking the computer.
So apparently I have a scheduled task called mcupdate_scheduled that woke my computer up. Next step, go to the start screen and type "scheduled" and you'll see an application called Scheduled Tasks. The path to the task is listed in the message above: Microsoft -> Windows -> Media Center. Navigate to that in the tree on the left and look for the mcupdate_scheduled task. If you look at the task and go to the Conditions tab you'll see that the "Wake the computer to run this task" checkbox is checked.
Seriously? Who in their infinite wisdom decided that Media Center update was so important that my computer needed to be woken up for it? There's even a KB article for it: http://support.microsoft.com/kb/979878