You're being downvoted - maybe because it sounds like a snarky answer? I've thought about your response and after some thinking, I feel that it's a legitimate answer. I reflected for a moment and have decided that I am starting to do the same - using the computer with the expectation that I am not the only user. It's paranoid, but it also helps me sleep at night.

Maybe working at a large insurance company, where every keystroke could be replayed by a perfect stranger, has rubbed off on my home usage. But I come from a networking field and know perfectly well that our little home routers are not things we should rely on to be on our side. Maybe they are, but I think there's a bit of luck. A lot of planning and a bit of luck.

I was being serious even if it came off flippant.

There is no way to be 100% certain of your security with a system as complex as a computer so if you need 100% certainty then you have to assume you are compromised, even if that is a very low probability.

Now that I think about it, that's the same reasoning Batman used to justify attacking Superman in the movie.

"preemptive self-defense" ftw

I wouldn't turn on a hacked system, except from an Ubuntu install DVD, or some other recovery disk.

Therefore the only secure practice (doing a combined approach) is to only boot from an install DVD.

Or alternatively: just never turn the computer on.