Windows Vista provides the means for extensive Parental Controls governing (on a per user basis) WHEN accounts can log on, what applications can be run, what type of content can be viewed and which web sites can be visited. You can choose to provide a list of allowed or denied sites and the affected user(s) can request additional access as appropriate. Parents can view an activity report too.
It's important to use these features in the way they were intended - that's as a way for Parents to become involved in discuss with their Children regarding how to stay safe on the Internet. Children need to learn about the dangers (ideally) with their parents to enable them to be safe on systems outside the home too.