I used to do this, I had to quit it really fast.
The thing is people somehow think that if you repair their computer one time that it all of a sudden has a lifetime warranty from you. Matter of fact I haven't taken a dime for a computer repair in 4 years and I still have one guy call me every few months expecting me to fix software issues on his computer for free.
It is the worst with the software issues, what you will do most of the time. Your average customer is an idiot when it comes to computers, and (as far as I have seen), is not teachable. You can clean up all their viruses, ad-ware, and useless memory resident programs (and perhaps even talk them into upgrading that 32MB of ram to 256 MB), install all the right safety programs and they will still have their computer ruined and unusable in a month.
I have gone so far as to lock down computers tight for people, make firefox the default, hide all reference to IE, make all IE shortcuts lead to firefox, etc, etc. Have active security programs running, etc. Come back 2 months later and they have 2 dozen ad programs running, 10 other idiot system tray programs, 3 different viruses and 5 or 6 separate AOL installations (when they don't even USE AOL).