Number (3) would have been my first thought since Windows may be interpreting every shutdown as a system failure. Next, try holding CTRL-S when you click the shutdown button which will force closure. Be patient because it takes a while... If your PC still restarts automatically, research the manual shutdown.exe command and its various parameters. Maybe something there will provide a key... Another thought -- are you virus free? If you are under control of an invader, that could also explain this behavior.