Since about a month ago, after many years, I assembled myself a new computer which is running Windows 7 now. It had the strange habit of waking up from the sleep-mode by itself at night. And I just could not find out why. It was not magic packets or the network card, it was not my wireless keyboard / mouse sending random signals (check
this link for info on that).
In the end, it was just the media center trying to update itself. This little annoyance can be turned off / changed in the Task Scheduler.