Not trying to turn this into an XP forum, but I recently bought a new Gigabit motherboard that used onboard Realtek LAN -- and had nothing but problems with it until the THIRD LAN drivers update was applied.
If you have Realtek on board, you will need to check your motherboard vendor site for drivers -- and you may have to end up installing different versions of the driver, one at a time, until the LAN finally works.