Turn the computer on. Immediately press and hold the Esc key for three seconds, and then release it. When prompted to, press the F1 key. The Setup screen will appear. The PC will have to be truly off (not suspended, or in standby mode, or in hibernation mode) for this method to work.
see if you can move round the BIOS using either the Tab key or arrow keys.
hopefully you can then enable USB legacy for DOS and the USB ports should then work and you could then use an external keyboard.
worst case scenario is someone has overloaded the USB ports and blown the none repairable fuse on the motherboard - the only way to get USB is then to use a USB card in the express port.
You have obviously had the machine apart is there a clear BIOS jumper on the motherboard?