First on the chopping block was the KVM switch. It's an incredibly well-built four port unit I bought back in 2000, and supports VGA and either PS/2 or AT keyboards and PS/2 or serial mice. There's only been one running system connected to it for years now so I had removed all the extra cables, making it effectively just excess distance in the lines.
One would think.
VGA moved over fine, but when switched to using the extra-long PS/2 keyboard and mouse cables, the system refused to boot past the initial POST. The cables are really quite long, twenty-five or thirty feet if memory serves; I got them from a few jobs back. I only need perhaps eight feet total in the current configuration.
After much fiddling, I ascertained that the PC was unable to handle such long keyboard cables. I ended up switching to a PS/2 to USB converter and ran a separate cable. A USB extension cable would a good idea at some point but is not required.
I'm now busily packing boxes to take to the electronics reuse/recycle store.