I spent a lot of time searching for a solution to a problem I was having getting into CMOS with my USB keyboard, which is connected to a KVM switch. All of the USB options were enabled in Integrated Perpherals, but the keyboard wasn't functional until after the boot sequence and DOS started up. The solution was to make sure the USB cable was plugged into USB port #1. I found this suggestion in a paper in the MS Knowledge Base regarding USB/Motherboard HID compliance. Regardless of whether your keyboard is connected to a KVM or directly to your computer, making sure it is plugged into port #1 may solve the issue.
I hope this helps some other poor soul like me.