You have a Windows Vista-based computer that uses a high-performance video card.
This video card uses multiple graphics processing units (GPUs).

In this scenario, the computer's graphics performance seems to indicate that only a single GPU is being used.

Nvidia & ATi were not the only guilty parties in the Multi-GPU issues, but Windows Vista was implicated, too. Microsoft just posted an update for both 32- and 64-bit versions, fixing issues where operating system would not pass instructions for the 2nd GPU to the driver, meaning SLI/CrossFire would effectively work as a single chip cards.

A supported hotfix is now available from Microsoft. However, it is intended to correct only the problem that is described in this article. Apply it only to systems that are experiencing this specific problem. This hotfix may receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next service pack that contains this hotfix.

Read Guru3D's GeForce 8800 GTS water-cooled SLI article