Free Parental Control Software
This awesome, free, package allows you to protect your children in a number of ways. On a pc level you are able to restrict the applications they can use and install, the folders they can access and even dictate the websites they can access. It has a built in website filter that will block inappropriate content and even prevent your credit card details being used without your knowledge.
For me, the most powerful module is the time scheduling. You are able to set, when your children are able to use the computer, by time of the day, day of the week and maximum hours per day, week etc. The same can be done for time spent online.
All this plus details reports of usage emailed to you, for free.