computer shuts itself off by itself

When you turn the power button on the HP pavalion 7845 it
will start windows and goes to different stages of the
startup procedure and then it turns itself off all by
itself. You have to unplug the tower from the wall outlet
and flip the switch from 220 and then back to 110 on the
back before the green light comes on, sometimes it you
have to do it 2 or 3 times. You then get a improper
shutdown and it starts to scan for errors. It has made it
through and shows no problems.