As a first suggestion, refresh the configuration data in your BIOS settings, then reboot the system, sometimes the low level DOS commands that read information from the hard drive or hard drives may not read correctly. If this does not resolve the issue, try entering safe mode, and remove the keyboard from the device manager, then reboot the system to get to the normal desktop (to see if reenumerating the system locates the generic kb driver).
My mother, (6 hrs away), keeps calling me about her computer. About every other time she goes to start it up, she has no keyboard. So we go through the thing of telling her to reboot and maybe it will find the keyboard and maybe it will take a couple of reboots before it finds the keyboard. It's the same keyboard that came with the computer (Compaq). She tried buying a new (cheap) keyboard, tried it, and the computer wouldn't even boot, (weird). Any suggestions on what is causing this, besides the obvious that her computer is 5 years old and slow as a dinosaur?

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