You can do that but it's not really setup for use as a firewall/router. There are distros out there specifically setup for use as a firewall router, eg Smoothwall:
http://www.smoothwall.org/
There are many others. With a firewall you want the minimal amount of things installed and running to accomplish the task. The firewall distros are designed for that purpose; mandriva is not. Actually, these days most people don't bother with trying to convert an old PC into a firewall router. The routers for sale for home use are cheap enough that it's not worth the hassle anymore. Most of them run a stripped down linux under the hood by the way.