How to monitor web sites visited on Internet?

Hi,
I am a mother of two teenage boys and would like to know
how to monitor the websites visited on a daily basis on
our family computer. I know how to check the history
folder but since they have learned that I know how to do
this, I believe they are clearing out certain websites so
they cannot be tracked. Can you please tell me if there
is any other way to get the addressess of the web sites
that are accessed?

Thanks for your help!

S. Duncan