Best bet? Make everyone else get their OWN computers! Chromebooks, like PB said, are cheap and fairly safe if you have wifi coverage. At the very least, have different log-in profiles if you're going to share a computer.

I use Avast antivirus on my main PC. (Norton has always been a total nightmare for me, so, nah). Porn? Chromebook, in an Incognito Window. lol But most importantly -- common effing sense!