that old pc needs a connection to the adsl and one connection to your network. so it needs the adsl modem and one network card.
if you have win98se you can use that as the internet sharing , but since you specified firewall you will need something else.
the 602pro lan suite. (which is free)
This is a secure mail server with antivirus and antispam, and a built-in firewall and proxy for controlled Internet sharing. The standards-based SSL SMTP/POP3 e-mail server provides secure encrypted communication. Antispam is provided by DNSbl service and a blacklist. It includes a Web mail client with WAP support. An integrated fax server provides unified access to faxes and e-mail. Secure your network with the built-in packet filter, and use the HTTP and SOCKS proxy with cache for Internet sharing. Manage IP addresses with the included DHCP server. The integrated Web server provides access to the Web mail client, remote administration, and user home pages. SSL, ISAPI, CGI, and FastCGI support is available.
New enhancements in version 2003 are a firewall with logging, support for Windows Server 2003, built-in BitDefender Antivirus, and separate IP filters configurable for the proxy, SMTP, WWW, and remote administration services.
here 's one way to setup the internet connection setting (use is as a basic template)
http://tinyurl.com/uh1j
here's another:
http://tinyurl.com/9gya
Leopold