I love playing devils advocate though, while geeksquad and firedog(circuit cities lesser known service center) are expensive, right off the bat a basic pc check up on an old unit is about 60 dollars, plus tax, then they move in like a mechanic. While not immediately 'fixing' the problem, they get the a-okay to fix it first then they do the work which is another 40 bucks or so. Even coming out to someones house is costly these days, but as midn1gh7 said, certifications and big names bring in more buiesness. more 'trusted' buiesness really, because some folks will allow you to fix their pc but turn around and say you did something wrong, kind of like malpractice.
If your serious about doing side jobs, try and get the work in writing or atleast the work you've done reccorded on paper or in your files to keep from something bitting you on the @$$