Web Access to Email

I have Exchange 5.5 Installed and I need a software that will enable me to access my exchange server from the web to access my mail. the exchange server is behind a firewall so using other web based email retrievers will not work. I can have access to port 80 on my exchange server from the internet through the firewall so i will need to install a software that will make use of the default page on my exchange server. Any ideas???