hey man, sorry to hear about your trouble with this. Not exactly sure how much help I will be but I could brainstorm with you a bit. First off, did you get your Garmin working with the steps you mentioned? I would check the BIOS. if your GPS took control of your PS/2 ports its possible that it may have disabled the other one. installing PS/2 support with the touchscreen drivers does this to the mouse port. It wouldn't be too far a strech to think that another device couldn't do the same to your keyboard port.
I think its safe to say that its not your keyboard. And I would venture to say that your mobo hardware is fine too.
I will do some searching and see what I can come up with. Worst case scenario you do a clean OS install, or at least switch to a USB keyboard solution... assuming your GPS is working now as-is.