i got IRQL_NOT_LESS_OR_EQUAL error after installing usb keyboard. i accidentally removed my old ps/2 keyboard while power was still on (10 seconds later and i wouldn't have had the problem at all i think). removing the keyboard while hot apparently caused the ps/2 port to fry.
after numerous reboots, googling and tweaking bios i found solution. i disabled the ps/2 ports from bios. now they don't use IRQs and there's no conflict. i'm using usb mouse also so i could easily drop both ports.