I have a Dell 1525 and for the past few weeks it has been starting up by itself. I use wireless internet. I checked the BIOS and don't see anything relating to an auto start up. I also checked to ensure my 'Wake on LAN' setting was off and unplugged my wireless modem for the day and came home to it still being turned on. I have run various virus checks (Latest Norton trial version and internet provider (Bell)) and neither has found any viruses.

Not really sure where else to check. Very random times to. Sometimes when I wake up it is on and other times it turns on during the day.

Any help would be appreciated!! 

Some added info: I am running Windows Vista and it only seems to happen when I have it plugged in. If it is unplugged and just running on battery it doesn't start by itself.