Right now I am just starting with this business. I have fixed or given maintenance to Windows PCs (and the occasional Mac) for more than 10 years.
I do not have any formal training or certification in fixing computers, but I am a game developer and multimedia producer, and as such my demands for perfomance and stability are essential to my livelihood. I have learned to keep my systems running at peak performance, constantly upgrading them both hardware and software wise.
I have also repaired and/or given maintenance to a vast amount of computers, from laptops to workstations, both for home use and for work.
I also keep myself well informed on the latest PC and mobile technologies, constantly reading hardware enthusiast sites such as HardOCP and Anandtech.