Is the driver for your ethernet card compiled as a module?
If so, it may not be loaded during startup.
You might have to add it to modules to autoload.
Here is an example with a e100 card. (change name to match your driver)
Code:
echo "e100" >> /etc/modules.autoload/kernel-2.6
I'm not using Mandrake anymore, so this location might not be the same but it is something like that. (Search for autoload in /etc directory)