Put the operating system into rom that requires 256 based encryption to reprogram. Eack box gets it's own individual code and the rom chips are embedded such that thay are inacessable without dismantaling the machine.
Now how are we going to validate the disenfranchised voter that signed their name "X"? They have no book learnin', and need an assistant to do the readin' and cipherin'. I see it at the polls every single time. "Honey, I am not asking you to tell me who to vote for. Just tell me which one had that nice commercial with the puppies."
I have a better idea. Put the operating system on a ROM chip which is visible through a window, bears serialized holographic seals from both parties, and is physically incapable of being altered except by replacement. Record votes on a write-once medium which is likewise visible, marked with serialized seals from both parties, and cannot be reused.
Any system which uses software on rewritable media, or which stores votes on rewritable media, is bad, period.