I will say this much, I had this kind of trouble on the ASUS P5N32-E SLI motherboard I had, with my Microsoft Comfort Curve 2000 USB Keyboard, thought it was the keyboard, but realized soon after getting my 975XBX2 motherboard back in March, that it was the motherboard causing problems with my USB keyboard. I'm still using my old, but reliable Microsoft Natural Multimedia Keyboard, which is a PS/2 keyboard
First try another USB port, if you still have trouble, then try it in another computer. If it still won't work, try new batteries if it's a wireless USB keyboard or try a new USB keyboard. If then you still have trouble, then you need to be willing to spend money for a PCI or PCI Express USB adapter, and finally if the USB card doesn't help, then you need to consider the possibility of replacing the motherboard.