Possibly you hibernated it instead of shutting it down.
Possibly you did standby.
Possibly your BIOS has "Wake on LAN" set to true.
Possibly it has one of the other "Wake on xxx" options set to true.
Possibly you've got a short in your power button.
Possibly you've got jumpers that are pulling off the motherboard, and it shorted and initiated the "on" sequence.
Possibly you've got GOOBLIEGOOBLIES in your house that want to download pr0n0graphies.
Or, also, you could have done automatic updates w/o realizing it. Sometimes when windows autoupdates, it thinks you really want to restart when you shut it down, because you're only shutting down to initiate the changes.
The LAN could be it, because I had the DSL modem off when I shut down and turned it on so that no one wakes me up early in the morning because "the computer is broken".
Thanks for the reply!! I never knew that a PC could just turn on though.
BTW, I have hybernate & Standby disabled so that gives more of a chance to the GOOBLIEGOOBLIES!!!
I never knew about that posting restriction. Yeah that would be pretty tough, but in other forums they sometimes give you a daily limit which can be worse...