Providing a positive externality

So it seems Skype's now likely to have built-in government surveillance back doors.

It really doesn't much bother me if people listen in on our regular chats with my parents back in Canada. But there's a positive externality to those who really need secure privacy if everyone who doesn't need it also chooses secure channels - it increases the amount of noise that a government agency needs to wade through before it's able to crack the channel in which they're really interested.

So, if I want to provide that positive externality, and I want a system my parents can easily install on a Windows-based system with no technical support, and that won't bog up their older computer or impose too-heavy of bandwidth constraints, what's recommended if anything?