I have an Abit KN9 motherboard, and a USB keyboard and mouse. The computer will not POST with the keyboard or mouse plugged in, but they can be used (and they are fully functional) if you plug them in later, after POST. On the POST side of things, it hangs when it shows the installed processor, and goes no further - pressing DEL to go to setup will not work. I've tried disabling legacy USB support as recommended, but can't find the option anywhere in the BIOS.
The solution to this was to perform a BIOS update which fixed the problem.
More on MajorGeeks: http://forums.majorgeeks.com/showthread.php?t=229762.