To be honest it might be worth thinking about upgrading to windows xp, if not you could install some kind of proxy sever on the 2000 machine. A good free one which (although looks complex) is easy to set up is squid (http://www.acmeconsulting.it/pagine/.../squidnt25.htm
) then when you wanted to stop internet access you would just stop the squid service on the main machine.
Hope this helps!!