One of my buds let me fix the laptop of a member of his bible study group. He was an older person who lent it to one of his kids. Now I have done this very thing myself and the results was the same, viruses.
Kids like games and music, only the sites they hit are dispensers of PC pestilence. This also happens when you hit certain advertisers and of course peep show sites. The need for user discretion is not always adhered to.
There is also a crime perpetrated on new PC users called selling an unprotected PC to make more money selling separate protection software packages. Many buyers are unaware they need protection, think they are careful, can't afford the cost of virus protection. My answer is because the viruses are out there, buyers should be informed.
Now, there are free anti-virus software available like Avast, AVG and others. Microsoft Securities Essentials, which I used on this laptop I fixed is also free. This was a Microsoft Vista laptop. When I got it, it had 264 or so copies of a Trojan virus. Just like the Trojan Horse, it looks innocent, packs a replicating mess. Two days of scanning and cleaning removed the viruses from the hard drive. Nothing but mindfulness will change the habits of the users that allowed this to happen, plus virus protection.
I don't know how well Microsoft Win 7 and 8 handle viruses, XP and Vista are wide open. You need protection, buy it or download a free anti-virus.
I am a Linux user. In my 20 years of using Linux I have not had any problems with viruses. Most viruses are written to attack Microsoft systems. I have had malware added but they were only compatible with Windows operating systems and could not run in Linux. I do have scanning and cleaning apps for Linux just in case.
Some users will fork over a PC to a techie friend and ask them to please fix it. This is not always a quick fix but a intricate tangle that could hose your PC. You could lose important stuff if the quick fix is format the hard drive, reinstall Windows. If the hard drive is large/full, there is no quick fix. Scanning files takes time, the more files the more time it takes.
For care-free, free-wheeling computing, you should do the following. Do regular maintenance (clean up files, stuff folders, defrag), do anti-virus update and system scan (can be done while you sleep), backup your newly cleaned hard drive to a remote USB drive or DVDs and create backup set points so that you will have a copy of "the last working system settings", just in case start up problems arise. These things will also help your techie friend fix your stuff. And just as you learn to check the oil and change a tire on the car, learn a little bit about the PC to save on a repair bill (read the users manual and or get a book on the subject).
Remember, popular computing without protection is a no no.