I've been building and working on computers since I was 10 and have had the reputation throughout my life as the guy that can fix anything that plugs into a power outlet... That's over 19 years of fixing people's broken computers. Sometimes it's frustrating to me how little people understand about their computers (and all other technology in their lives).
Nothing surprises me anymore about what people do to their computers, but the most common problems I deal with are "my computer's gotten slow," and malware infections from emails, thumb drives, and dodgy websites. (These often go hand-in-hand.) Of course my personal computer never has problems like this since I'm a control freak and nothing runs besides my OS at startup. I don't even use AV except to scan promiscuous thumb drives. I haven't had a significant hardware failure or malware issue since 2002 when a couple of RAIDed Maxtor drives burned themselves up. 😛
I can't blame people for not understanding their computers since nearly all hardware/software is designed/written by engineers for people that think like engineers. I think like an engineer, but most people don't.
The irony is that software which is written for the average non-engineer (such as Office 2007 and MacOS) are *loved* by my non-tech savvy friends, but completely confound and frustrate me. >.<