After a very long time, ASUS Xonar soundcards have finally a WHQL certified driver for Windows 7 / Vista, version 18.104.22.1683, dated 10/28/2010.
You can download the updated packages from the links below:
Xonar Essence ST
Xonar Essence STX
WHQL drivers for HDAV1.3 models are NOT available at this time.