The TiVos that have internal power supplies have a fuse on the power supply board in the incoming 120V AC line, but no fuse on the motherboards as far as I know.
TiVos with outboard supplies probably won't have a fuse on the incoming +12V DC line, although there might be "something fusible"* inside the outboard power supply, in which case if it blows you replace it by getting a new outboard power supply.
*Kind of like a "fusible link" in car wiring.
"I am altering the deal. Pray I don't alter it any further."
Darth TiVo, 14 February, 2011