BoatCop had a very good thread about this exact subject a while back, only he talked about stupid things parents do as well. For example, every day I see people who have their kids' first and last names plastered on the back window of their cars as well as the school name, sport/team and player number. Now a complete stranger knows where your kid goes to school, what his name is, what sport he plays and even what number he is!!!
As far as the computer thing, they have several programs that will track keystrokes or passively listen for certain words. IMO, when you are responsible for a minor and said minor is living under your roof there is no such thing as "privacy" when it comes to you protecting them. The only rights they have are to food, shelter and clothing. That's not saying you should strip search them daily, but under reasonable suspicion I don't believe it's wrong to go through any of their belongings to find the truth. It might save your family or their life.