Did you download and install the following Realtek 8111C LAN Controller driver from Realtek's Download site? It was last updated 9 days ago from today.
Win7 and WinServer 2008 R2 Driver Version 7.048 (Updated: 2011/9/19)http://www.realtek.com/downloads/downloadsView.aspx?Langid=1&PNid=13&PFid=5&Level=5&Conn=4&DownTypeID=3&GetDown=false
The Realtek RTL8111C LAN Driver on ASUS' website is from July 20, 2009.
Did you try a different LAN cable?
Are any of the LEDs for the LAN port flashing?