What does poking with a stick mean?

The difference between Windows Calculator and Excel is that Windows Calculator is designed for interactive use only. It performs calculations only when you push its buttons. Whereas Excel is programmable: it can be instructed to perform a long series of calculations without needing user interaction for each one.

So Joel is analogising pressing the buttons in Windows Calculator (which is what you have to do each time you want to make it calculate) with poking things with a stick. I think the intended image is of someone poking an animal (or, as suggested by petemoloy, a stuck mechanism) with a stick in order to make it move.

We use this expression in our shop pejoratively to mean a system that can't be trusted to work on its own without manual intervention. I imagine an old steam-powered machine consisting of wooden cogs that an old time engineer has to literally poke with a stick when one of the cogs sticks.