http://vista.blorge.com/2007/07/22/microsoft-gets-nvidias-sli-and-atis-crossfire-working-in-vista/
A stupid issue with SLi and CrossFire has now been fixed. Direct
download link at Guru 3D (provided in the above link as
well).
Or if it's gone for good (you know MS, they are fast to kill)
http://downloads.guru3d.com/download.php?det=1680
Download the fix from the above link.