I actually do quite a bit of "free-lancing" in the computer repair/maintenance/teaching business. I have been working part time at a computer store for a bit over 5 years now and have picked up lots of good skills and knowledge. I have no certification of any kind, and I just rely on word of mouth from those whom I have helped for my "advertising". I only work on the machines of folks I know well, however, for two reasons: 1) I don't have the free time to do it very often, and 2) I know I can trust them to not come back on me if their machine is a lemon and I can't fix it (they trust me, too, so that helps )
As long as you don't advertise that you have any kind of degree or certification that you don't have, you should be just fine.
An inlaw of mine spent a pretty good fee to a guy to clean up his puter. He ran several programs that most of us are accustomed to running to do this. I was wondering about the feasability of doing that myself. Or would I need to have credentials as a real fix-it man?