In general, if I know that someone who is a 'general purpose' (read: facebook, msn, youtube, music) computer user is going to use a system, I try to make it as "idiot-proof" as possible. Chrome installed, IE hidden, Adblock installed to keep them from clicking on any ads. Limited account if possible. It seems to work pretty well.
My girlfriend is slowly learning how to not destroy her computer. All she uses is stuff like skype, msn, facebook, and web browsing; and I haven't had to fix her system in quite a bit. However, it likely helps that I set up an old Pentium 3 laptop with Lubuntu for when her friends are around. Limited account, Chrome/Midori and emesene, all labeled as Internet and MSN. They can hardly tell the difference. And with the limited account, it's nearly impossible to kill.