Its interesting how people are very intimidated by computers. Can you remember how you felt when you got your first computer? I can, but luckily because of my work I've worked on computers for 20 years, so I don't feel the slightest intimidated with them now.
As for putting a card in a newsagents window, I probably could, the trouble is I personally think its more trouble than its worth, after all you can fix someones computer today and something else goes wrong again tomorrow, try explaining the problem in any known language that the person will understand, they will think you're pulling a fast one.
I think its very difficult to justify your time when you spend 2-3 hours in someones house resolving a problem like virus infection.
I've seen these cards, but I've no idea how good/bad they are. Still, its an interesting article.