My motherboard is a BIOSTAR TForce 6100-939 with an NVidia chipset.
Your board [http://www.biostar.com.tw/t-series/products/socket 939/tforce6100-939/product_details.php] has the GeForce 6100 and NForce 410 chips.
My board [http://za.asus.com/products4.aspx?l1=3&l2=101&l3=0&model=1138&modelmenu=1] has the GeForce 6150 and NForce 430 chips, just newer revisions of your chips.
On the tech support page [http://www.frontosa.co.za/support.htm] of Frontosa (a local hardware distributor) they say: "Asus A8N-VM does not function with Sunix".
The specs of this board [http://za.asus.com/products4.aspx?l1=3&l2=15&l3=0&model=768&modelmenu=1] show it uses the same chips as your board, GeForce 6100 and NForce 410.
It may be an incompatibility with the hardware, but to rule this out I would suggest installing a fresh copy of Windows to another hard-drive, or another partition and see if the card works there. You may just have something screwy with Windows, as MTNDD decribes in a previous post:
with failures we’ve had to re-image the machines once we’ve done that it worked fine