Using a Proxy server

I have a mixed bag of WinXP and MAC users with a large turnover where not everyone logs into the domain but can still have internet access.

I have a W2K server as the DC and ISA server 2004 installed on a Win2003 server.

I would like to force everyone to go through the proxy whether they log into the domain or not. Is there a way I can do this without having to manually configure everyone’s PC or MAC and ensure that these settings are kept even if a user removes them?

Thanks