First try going to (right click) My Computer > properties > choose devise manager > click on network adapters > right click and choose update devise driver. Windows will ask you if you have the file > Yes > point to the drive and folder. Usually Windows can find the right file in the embedded folders, but you do have to unzip it first.
If the Windows update method doesn't work you can run the program directly from the install file after you find it!
Asus has some very confusing LAN drivers, for those unfamiliar! After unzipping the file there are many embedded folders. In most cases the first "Asus Setup" file will work, the 1KB files can be ignored, the first 461KB file brings you into the program on my Asus board. You probably can't screw anything up, the files that don't work usually don't do anything, so you don't have to panic, you should be able to find the right one!
You can put the downloaded files on your HD, almost anywhere, the program will install it in right place. You can install from a USB stick too!
The file will look something like ( on mine ): Realtek_LAN_V5782_6247_743_XPVistaWin7
Get them from Asus only!