Looking for free pc monitoring software

I am currently helping a charity that provides training to teenagers.
They are currently a peer to peer network run via a broadband router.
Any help for 2 problems.

1. Looking for a way to block access to adult sites that are not rated as unblocking all the sites they access to is taking a long time.

2. A way to remotely view and control what they are doing. Normally I would use XP remote access but they are using a number of versions of windows.

Any help would be greatly appreciated.
Please though all software suggested must have a free license as their is no money to buy them.