Are there any free programs that can record what keys are entered?

I'm concerned that my two teenage boys are doing inappropriate things on the computer and would like a program to record what they do. I've heard of this Amac , but after reading some about it, not sure I want to install it, also, it's very expensive. Are there any free programs that can record what keys are entered? If they can do other stuff too, I'm sure that that would be great.