Have a look in your BIOS - follow the on-screen prompt and press the appropriate key such as F2 or Delete to enter the "BIOS" or "Setup" screen literally as soon as your PC switches on. There will be instructions somewhere on the screen (usually at the bottom) with how to navigate around the BIOS menu. See if there's a screen with options for USB legacy keyboard support. If there is, switch it on. I'm not exactly sure what that does but it sounds like it makes your PC think it's a normal PS/2 keyboard when you want to use DOS, Windows 95 or Safe Mode. I could be wrong - just a guess and it's worth a try
Let us know if that helps