What output do you get when you run
Does it show the rtl8139 or 8139too modules loaded? If not try loading one of them by hand (using
modprobe rtl8139) and see if that works. If it does work (i.e. you don't get an error message) then it looks like for some reason your module is not getting loaded at startup - you'll want to find the relevant config file (not sure exactly where it will be in your disto - I'm from the Slackware camp - try somewhere like /etc/rc.???/???/) and put your modprobe line in there, on Slackware it would be either /etc/rc.d/rc.modules or /etc/rc.d/rc.netdevice but it varies per distro - hopefully someone else on the forum will be able to tell you the correct location.
HTH
Jamie...