|remove mouse and anything else that is plugged into USB ports. What about keyboard? If you are using a wireless keyboard, you have a USB gongle that will need to be removed. If you are using PS2 keyboard, you can leave it plugged in. If it still does not start, you will need to open the case (desktop assumed) and look for internal USB connectors to the motherboard for front USB ports and internal USB hubs (front or back) and remove those for the test.|
Try booting, if it works, add in keyboard, etc. one at a time and test. At this point, update motherboard drivers, then shut down and add in one internal plug at a time until it no longer boots, that will be the problem piece of hardware that should be replaced.
Also, while in the case, tip the case a bit and look for loose screws rolling around inside of your case that could short something out.
You have to be a little bit crazy to keep you from going insane.