Assistance with small network setup

I have setup a local network at home.
I have ADSL working and all computers communicating to the Billion router.

I require assistance setting up some specifics.

Ultimately I want the following:
1-Xbox 360 and Wii connecting to the Internet. (Have this working now).
2-Home Theater PC connecting to the network but have all connections to and from the Internet blocked.
3-Laptop connect to network and Internet, including P2P. (Laptop is running Ubuntu).
4-Second PC connected to network, and access to internet only when I enable it. No connection by default.

I am having some issues with number 2 and 3 mostly. The HTPC has never connected to the internet and I never intend it to, thus I do not have a firewall or anti-virus software on it.

First question: How do I block all incoming and outgoing connections to the Internet on this PC (Windows XP SP3) while maintaining connections to the local network (via wireless)????

The second issue is that when I enable the firewall on the Billion, my P2P gets blocked. I am unable to find where it is getting blocked in the firewall.

Second question: Should port forwarding bypass the blocks on the firewall, and if so where can I get some good information about setting it up????

For point 4, I have Trend Micro 2008 installed and figure I can 'halt all internet traffic' by default with that, then allow it when I require the Internet (about once per week for an hour).

Last question: Is there anything I should be wary of, or anything else I have forgotten????Start Free Trial