Do you have latest BIOS installed? We had an Acer that did this in middle of night, I kept telling kids off for not switching PC off. Only came to light after a guest satyed in same room as said PC and complained of being woken in middle of night by sound of PC booting! Updating BIOS fixed problem.
You don't say what operating system you are running. There are reports of Spyware Doctor waking Vista machines from sleep and hibernate.