In my case, I was using a KVM switch. I could not get the mouse to work but the keyboard was working fine. I changed the mouse and reinstalled the i8042 drivers from the NT Server CD. The keyboard and mouse started working properly.
I got this error using an MSI motherboard K7T266 Pro2 with AMI BIOS without a KVM switch and with several different keyboards. This error started to appear after I enabled in BIOS "port 64/60 emulation". The manual indicates: "This field within the function "Integrated peripherals" allows you to enable or disable the USB Port 64/60 Emulation function. When the function is enabled, the USB keyboard is allowed to type some special combination keys”. Be sure set the field to ''disable'' to avoid the warning on PS/2 keyboards.
Message usually generated by computers connected to a KVM switch (to be more specific, a poor quality one). If no KVM is used and if the keyboard is still working correctly then no action is needed. Otherwise, check the connections. If you still get this message, try using a different keyboard.