Quizzes and quandries

First of all, the incredibly difficult answers to last week's quiz. Mosh is the hands down winner, but there are some that even he failed to spot.

• Celsius to Fahrenheit = Multiply by 9, divide by 5, add 32
  
• Watts = Volts times amps.
• Litres to gallons = multiply by 4.54.
• Kilometres to miles = multiply by 5, divide by 8.
  
• Kilograms to pounds = multiply by 2.2
• Volts = Current divided by the resistance (No Mosh, V=I/R)
  
• divide by 1,048,576 = bytes into megabytes
  
• 4 weekly into monthly = Multiply by 4.34 (bonus points for having the time to do this one.)
  
• The complete bollocks that is the Body Mass Index = weight divided by height
  
• Mach into miles per hour = multiply by 761

And secondly, I apologise again for failure to post. I got in last night, and John was decidedly tearful and very upset. He'd tried to fix his girlfriend's PC, and plugged my hard drive into it. The PC had a virus, and as soon as he plugged in my (very very important) hard drive, it decided it was going to delete half of my contents. Fortunately, he'd had the foresight to say "ok, I screwed up" at this point, and he didn't try to repair the situation himself. I downloaded a great little free program called "undelete plus" which I then ran for nigh on the next 3 hours, undeleting all the programs and files that had been deleted by the virus, some 53,000 files to be exact.
But, all's well that ends well. Nearly. I had banned John from his PC because he'd left the backdoor unlocked all weekend and anyone could have got in. He'd been given a list of 4 things to do on Saturday before coming to meet me at work, and that was the top of the list and most important one. I knew, in reality, that all he'd done on Saturday was play on his PC and this is why I thought it better to stop this addiction again before something really comes to harm. So to quell his need for a fix, he'd decided to do this other PC. He'd also been on my PC, and if this virus had got onto mine, I cannot bear to think what the consequences would have been. I stayed perfectly calm with him, I spoke to him about it last night, and again this morning. He needs to understand that whilst he'd done wrong. he'd done the right thing by not trying to fix the problem, in turn making it worse. I also told him that he would be banned from any pc in the house, even if he just wants to listen to music. I do not want to impose passwords on everything, but if I do have to, then I have to. Has anyone got other suggestions to stop this from upsetting him but ensuring that he understands his actions?