Ny computer has developed the habit of 1) Keeping a fan running at low speed for ages once the operating system has shut down ( now XP pro but was the same in 98). If you hard switch ( at the back) the machine off it becomes necessary to pull the power chord if you want the computer to start. I'm sure it's a motherboard command - but which one. I have cleaned out all the dust in the case in case we were overheating.