My PC is an HP 9400 workstation that's just started misbehaving. Will only start when mains turned off then on, and comes on immediately. All fans start, but nothing else happens and screen stays black. I then turn off by depressing power switch, but won't switch back on however long I wait, and I have to do the mains thing again, with exactly the same results as already described. I've changed BIOS battery, and old one was dead, and reset BIOS at the motherboard switch, but this hasn't made any difference. Any ideas what my problem is. Denis