stay away from usb... such a bad invention lol...
I know you said it is an old system, but use a program like CPU-Z or Everest on it, and post the specs on it, or if you know them yourself do it that way, it may be something as simple as the version of windows you are trying to use in conjunction with it, if it is too old, and keyboard and mouse are brand new, with like the media stuff on them, then that could be a problem, and we won't even get into how usb works with old systems LOL