If you reboot and press F2, F8 or F12 before Windows starts to load, does the keyboard work?

If it doesn't respond to any of the function keys before Windows loads, you can try this:

Power off and unplug

Press/hold power button on tower for ~15 sec

Open the case and remove the motherboard battery

Press/hold power button again for ~30 sec

Leave it unplugged like that for a couple of hours

Then reinstall the battery (right-side-up!) and see if it will work now with only mouse, monitor and keyboard attached.

Ron