Use a simple old bulbous Microsoft or Compaq mouse PS/w mouse and PS/2 keyboard... Remove all your USB stuff.
If that doesn't work, use one of the Basics tricks.
Remove all but the power supply, one memory module, and video connection. NO hdd, no optical drive. Boot to an MS-DOS or Windows 98 Boot disc. Does it run? That tells you something. Change memory module, try again.
The idea is get it running with just the basics of DOS or Windows 95 or 98 on a floppy, then add a component, until you find the point where it no longer works.
This will also help you think things through that you may have missed with a more complex setup.