Theres a lot of info on this topic, just search the forums. But let me summarize what I have seen:
1) On a multi user computer, the best way to compartmentalize is setup multiple users and assign permissions accordingly. E.g. in your case, you could be the sole administrator and the younger members could have standard accounts. That way, you can restrict access to drives, folders and files and control what users can do via group policy and security policy.
2) You can combine parental controls with the family safety filter (freeware) from MS to control access to websites. Its a bit limited but does an adequate job.
3) Third party tools also have parental controls. E.g. Norton has a parental control software called Norton Online family, also I think its other products Norton 360 and NIS have parental control features. Somebody the other day mentioned a tool called K9 web protection. K9 Web Protection - Free Internet Filter and Parental Control Software | Free Internet Filtering and Parental Controls Software