my pc turns itself on

O.K. here it goes
after I select ‘shut down’
from the start menue.
the screen displays “it safe to turn m/c off”
and automaticly powers off.
then at random either with one or two days
and at any time, I hear the fan running
on my computer andthe m/c is on. like i
hit the power button on the front of the m/c.
and i know it was off and quiet before i left
the computer room, i was blaming the kids
for leaving the m/c on.
i have file sharing and all that checked off!
joe