I found 18 keyloggers on the computer my 15 yo uses for his school work.  I can delete them, but he will just download them again.  I know how to block the sites if he uses Internet  Explorer, but he uses Mozilla too.  How do I block them on Mozilla?

Because much of his schooling is online, passwords protecting parent only functions may have been compromised.  I have changed many of my passwords, including access to the computer accounts.  Ds is not allowed on any computer unless he is being watched.  Everyone else has been instructed to log off their accounts or switch users  when no one is in the room, which is a pain because  Dad, 19 yo ds, and I are the only ones who know the passwords for all accounts on all but one computer.

Right now he is only allowed on the computer to do school work that must be done online, and is only on the computer where I am working.