Try checking your BIOS settings. If your board has an onboard video chip it may be conflicting with your Card. (Doesn't help that 3dfx went out of business).
Check that the video IRQ is assigned automatically and that if it is AGP you have assigned the AGP port as the primary display and not the onboard device. If that doesn't work try disabling the Video BIOS cache, VGA palatte snoop or DAC snoop. Hope that helps.
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