I would say charge a flat fee of $20 for the first hour, and $5 for every 15 minutes after that. That way you are guaranteed to walk away with $20, and many problems can be fixed in one hour. If the computer is really hosed and you have to get and install part, and/or re-install the OS, you may then want to negociate with them for a fair price.
One last thing... I installed Ethernet cards in the dorms for free. It was a great way to meet the people when everyone moved in at the beginning of the semester, especially hot freshman girls. Of course, then you become the computer geek to them, but thats OK. I met a girl who I dated for 3.5 years when installing an ethernet card.
Ryan