Connect two computers directly through Cable/DSL Firewall

[joopa]

Hi,
I have a Linksys BEFSX41 Firewall Router. I want to connect two computers through the Firewall, Computer A connected to the Cable/DSL Modem port, Computer B connected to one of switch ports.

I want Computer B to access and write data to Computer A and preferably let it use Computer A to reach another network through a secondary NIC on Computer A. I Don't want Computer A to access Computer B.

I need this for extra security purposes.

I tried several setting but can't get it to work.

Any suggestions!

Thanks a lot.