If they're USB, then you probably connected the USB wires incorrectly. If they're PS/2, then you may have a bad motherboard.
I am having a problem. I just bought components to build a new computer and I put it all together but it will not boot up. There is power going to the motherboard, I know, and to the video card, hard drive, and all the fans in the case. However there is no power to the keyboard, mouse, and CD/DVD Rom drive. I checked all the conections and they seem to be fine. If anyone could help me out with this I'd appreciate it. Thanks!