According to the Microsoft web site, the problem can occur on Vista-based systems running multiple "high performance" GPUs. The problem occurs when Vista fails to forward driver-render requests to the secondary GPU, resulting in "the computer's graphics performance seem(ing) to indicate that only a single GPU is being used." The issue shows up in all versions of Vista.
Multiple video card setups are becoming increasingly common in gaming systems as gamers seek ever-higher levels of performance to accommodate the latest releases. SLI (for Scalable Link Interface) is a multi-GPU technology developed by nVidia that allows users to link multiple video cards for a single increased-performance output; ATI has countered with a similar technology built around its own cards known as CrossFire. Old-school gamers may recall the original SLI, Scan-Line Interleave, a multi-GPU scheme developed by 3dfx for its Voodoo2 cards.
Microsoft cautions that this hotfix may receive additional testing, and is only intended for users experiencing this specific problem. More information is available from Microsoft, and the update may be downloaded directly from Guru3D.