Hi WRL,
I am unaware of a program that will do this (but I would of thought a few of the server OS would allow this) Richard or E - Liam would probably know.
If you bought a router, you can have some control over access times and viewed material and restrict adult content to one PC etc....
You would probably get faster speed on your "client" pcs when connected to the internet as well.
I use a Netgear DG834G which has some faclilities as mentioned above. Someone may be able to suggest a router with better control facilities.
Hope this helps,
F - Wyg