1. Other people.
2. Wake on LAN, Modem, etc settings.
3. The BIOS has that feature to wake at a scheduled time.
I have a non-branded PC that my brother-in-law put together for himself and the passed it over to me. It has a AMD x52 chip and runs on XP Pro.
I have a constant problem of where it boot's up everyday mysteriously. I always shut it down completely in the evening. Without fail in the morning it is running. I have the Mcafee virus and spyware scan but doesn't seem to pick up anything. If I shut the main switch behind, then it does not start up on it's own.