1. Is C still a viable language to learn to write programs for PC/MAC Ideally in Win MacOSX (cocoa looks nice) and Linux or should I be looking at C# / Mono / QT / ??
There's 2 programming languages you need to learn. C++ (Treating C as a subset of C++ here), and everything else. Once you've learnt C++, the "Everything Else" comes naturally and without much effort, except for godawful things like lisp and haskell, but those are rarely used outside of academia.
It seems that all later programming languages seem to be adding to C (extend .. embrace ??) so Would C be a good start place ?
No, do not just learn C, as a lot of 'next gen' languages are Object Oriented. If you only learn C, you won't learn about that, and be in for some strife if you do try and learn C#/Java/etc. Find a good C++ course, and get stuck into it. Having said that, I've no idea where you'd find such a thing. Teach yourself would be my advice.