Several coworkers and a couple of my wife's friends used to think of me as their "on-call" computer technician. After spending many hours, and even entire weekends trying to correct their computer issues, I started telling everyone that I had no clue what was causing their issues. Instead, I would refer them to the Geek Squad or the OEM. If they persisted I would offer to install Linux on their computer, which no one ever took me up on.
The vast majority of the time I found spyware, adware and viruses from their kids using P2P networks. Expired trial anti-virus programs from when these computers were new seemed to be the norm. I also found several infected computers that contained a lot of temporary internet files and cookies from porno sites. This was awkward, especially when I knew that it had been the husband that visited these sites. Happily, no one calls me for computer help anymore.