If you go into System Settings-->Network
, highlight eth0, click edit. Under the general tab you will see a checkbox by Activate device when starting take the check out of here, you might want to place a check in Allow all users to activate and disable device. Now it won't try and start at boot up. Once in to Fedora you can then go to System Tools --> Network Device Control
, highlight eth0 and click activate. It will then activate your eth0 without asking for roots password. You will have to save the changes and restart the Network service for changes to take effect.
In Fedora Core 2 the command is system-config-network