I've actually never charged a single dime to ever work on a person's computer for them (although seeing how easy it is kinda makes me wish I had...)
I live in a fraternity (funded by none other than Paul Allen) and trust me, you think you've met people who are idiots with computers, these morons download pirated software and porn, then scratch their heads when their computers break down.
But being the nice guy, I always reformat and reinstall everything for them, for free...
Plus I do that for all my buddies from back home and their parents, and my parents, and their friends...