The solution....
I worked out the solution to this years ago. Charge the beggers. I charge £25/hour. Virus sir? You need a Windows reinstall - 2 hours = £50. Why should I charge less than I get for my day job? And only doing the work when I really actually have time helps too. Drop it over but I don't have a free slot until next Wednesday...
The word fairly quickly got around that I don't work for nothing. I don't get asked very often, but its fairly nice pocket money when I do.