try using a older version driver.
I'm assuming you used the driver supplied wth the card, and then you grabbed the latest drivers from the site. so try some of the ones in between.
I have a all in wonder from visiontek and the original drivers sucked on win7, and the latest one would bluescreen my pc.
the driver version beloe the latest works but once in a while using mediacenter it'll freeze.
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I got your salvation b*tch

wanna go night night fool.