I use to experience this heavily. I've resorted to self-deprecating humor to tell them to buzz off without them feeling too insulted.

"Oh wow, [Windows|OSX]? I don't know how this stuff works, I only work with little computers!"

With my parents I feel less obliged to be polite so I generally just say something about not bothering a mechanical engineer with your car problems. The idea there is to emphasize that despite a degree in something they think is plainly related to installing printer drivers in windows XP, that is not actually something that I am trained in.