Hope u have SP2 and latest updates installed....
Well u may Reinstall / upgrade the network drivers...
Also some time punching the nerwork cable can be faulty... make sure u Punch the Network cable (CAT 5) with the CAT 5 socket properlly...
do one more thing... just right click on the network connection ICON (LAN 1 or LAN 2 whatever) Go to Properties then at the Advance TAB look for Connection Type ... try with the different settings available...
P.S. is it Network Error 05 or 65
take a look here