You could always use a DNS service such as OpenDNS, You can apply the settings either on a computer by computer basis or on the router, in which case anything that uses the internet connection will be protected against. It's totally free to use and if you make an account you can see reports, customise messages and also apply inidividual rules for blocking specific websites or catagories of websites. I have used it at many different homes and showed the parents how to use it with great success.
http://www.opendns.com/
Jase