Yep, it's amazing how many people start by assuming that their OS is safe... and there's no basis for this assumption, even without assuming intentional backdoors baked in, there are some many 0 day exploits for all the software we use daily that...
...heck, I'm not even 90% sure that compilers don't add backdoors to software (it's not paranoia... but with a 30 y o idea, you can imagine that people have had time to refine it to unimaginable subtlety :) http://cm.bell-labs.com/who/ken/trust.html)
The only alternative to making this assumption is to treat computers as compromised non-trustable entities, forgoing their use as true extensions of the mind and leaving all individuals at the mercy of the ever-growing computing system. Instead, we treat the assumption as fact, making do with the imperfect OSs we have and incrementally fortifying them to solidify the assumption.
...heck, I'm not even 90% sure that compilers don't add backdoors to software (it's not paranoia... but with a 30 y o idea, you can imagine that people have had time to refine it to unimaginable subtlety :) http://cm.bell-labs.com/who/ken/trust.html)