You should be able to use the good ol' linuxconf in X to set the DHCP to run at boot. If not you may ave to edit one of the config files manually like net.config(or something(don't laugh at me)).
You might even be able to upgrade to Mandrake to 7.2 which has a nifty little cheater GUI that lets you setup DHCP. I connect using DHCP for my DSL and this took me about 2 minutes to setup and get online after install of 7.2.
Jasutinsama<--dont listen to him he is just a newbie