computer is connected with an ethernet cable to a cable modem. But all I get is a message telling me that there is a self asigned IP address, and I am unable to connect to the internet. I am running 10.4 Tiger.
This is probably a problem with your cable modem, not your computer, but check to see if DHCP is enabled on the Ethernet connection, it should be trying to automatically get an IP from the modem. Next, try to renew the DHCP lease. Lastly, see if there's a way to connect to your cable modems configuration settings (maybe via USB?) and make sure it's trying to give out IPs using DHCP. What kind of cable modem do you have?