OK, I'm sure we can help to sort you out, but first, can you tell us what the spec. of the "old PC" is?
Ubuntu is a fairly sophisticated distribution (distro) and although not as resource hungry as Windows, still needs respectable hardware to run properly.(and this has all the hallmarks of an under-powered machine).
If your machine is under-powered, there is almost certainly another distro that will run effectively.
One initial final thought, are you sure it is a software problem, rather than hardware? I don't mean in terms of the mouse or keyboard, but perhaps the motherboard USB controller itself.