How old is the motherboard? If its more than 2 years it wont run at 400mhz. What processor did you originally have. The best you can probably do is 333. You need to reset your CMOS. There should be a jumper next to the silver coin battery. Unplug the computer and press the power button. Then move the jumper to the next position for a few seconds and then back to where it was. The plug in the computer and turn it on. The motherboard should be at default settings.