I set up my laptop a while ago so that if the kids surfed the internet they wouldn't be able to find anything dodgy. I also stopped any 'pop ups'.
While on here tonight I have received one dodgy popup and - looking at the google page (it was set up with a password and a distinctive circle pattern so that you could tell the security was switched on) it's back to coming up with all the not-so-nice stuff if you search for it.
I don't know how it got switched off - don't think I could remember the password myself, and not sure who would have switched it off. I have a feeling it's gone and 'reset' itself. Can this happen?