I have Asus S939 server board with dual Broadcom NetXtreme BCM5721. The drivers for this are identical to the drivers for your BCM5702 chip - I compared them with a CRC check.
I was running driver version 10.62 and have just updated to the current 10.78 drivers. They should work for your board too.
Download the 32 bit file win_xp_2k3_32-10.78.0.0.zip from here - the XP and Server downloads are the same
http://www.broadcom.com/support/ethernet_nic/netxtreme_desktop.php
Unzip & install through Device Manager - it should work.