Are you looking in the right place on Asus's site? I dont have the serial no. of your board (for look up), but for the M4A785-M there are drivers for everything from LINUX, WIN 2K on up thru XP, VISTA and 7, both 32 & 64 bit flavors. The link for the various OS drivers for the LAN is seen in the available downloads.
Personally I think that you havent got the right spot on Asus's site for the download. Some tricks also are to look at the LAN chip of your MB and compare the specs. of adjacent Asus MBs model numbers using the same chip then inspect the support availability for those adjacent MB's. Also (like in the case of Fujitsu), check to see if the manufacturer has sites in other countries. For example, Fujitsu's Singapor site often offers drivers for additional OSes not offered or supported on their USA site. (Probably has to do with different technology emphases associated in different countries.)