Alright. Sorry I didn't notice that both were using USB.
Has your USB keyboard worked before when connected to this system, from power up?
Most likely, your PC isn't detecting a keyboard connected to the PS/2 port so it thinks there is not keyboard present despite the fact that you have a USB keyboard connected.
Go into your BIOS and see if there is an option like USB Legacy Support or similar. If there is, make sure the option is enabled.
If there is no USB Legacy support, then it's likely that you won't be able to get past the Power On Self Test process, or have the USB keyboard work, without a PS/2 keyboard connected to the PS/2 port...unless your system has booted successfully before with just a USB keyboard connected, and in this case it's likely to be your BIOS settings.
|Desktop PC| |Macbook Air 13" (Mid 2013)|
|Windows 8.1 Pro (x64)| |AMD FX 4100 Black Edition 3.6 GHz @ 4.0 GHz OC| |Arctic Cooling Freezer 13 PRO| |Gigabyte Nvidia GeForce GTX 560 Super Overclock 1GB| |ASUS M5A78L-M/USB3 Motherboard| |Samsung 840 Pro 256GB SSD SATA III HDD| |Kingston HyperX 8GB PC3-14900 DDR3-1866| |OCZ ModXStream Pro 700W Modular PSU| |NZXT Vulcan Micro ATX Case|