I had this problem for a while already, every night, exactly at 12:00 am (00:00 hs) my PC turns on by itselft.
I disabled every option for Wake Up in the BIOS (Wake up on lan, wake up on timer, wake up on keyboard, etc) but the problems continues.
I also updated to the last bios version from Gigabyte.
I always hibernates my computer instead of shuting down the OS (Vista Ultimate 32bits).
Any ideas? it has to be the bios, am i right?