Usually, when I go to sleep, I also put my desktop computer to sleep.
But, for the last couple of months, the computer would always wake up by itself and I couldn’t figure out why.
With the help of Pete I was able to find out:
C:\>PowerCfg -LASTWAKE Wake History Count - 1 Wake History  Wake Source Count - 1 Wake Source  Type: Wake Timer Owner: [PROCESS] \Device\HarddiskVolume2\Windows\System32\services.exe Owner Supplied Reason: Windows will execute '\Microsoft\Windows\Media Center\mcupdate_scheduled' scheduled task that requested waking the computer.
Using PowerCfg –LASTWAKE it is possible to know what woke up the computer the last time it woke up.
It turns out that it was a Media Center scheduled task that was waking up my computer.