I think Comodo parental control [CPC] would be a very good application to develop. Which can help control what users do and how long they can be on there username or the internet. There is other things that can go in parental control write what you think on this page.
theres something like that in vista…it might be good for xp
Something like Crawler Parental Control ( http://www.download.com/Crawler-Parental-Control/3000-2162_4-10549693.html ) and System Protect ( http://www.system-protect.com/ )