My friend has an alienware laptop with a ATI Mobile Radeon X1600 card, but when doing the performance index, an error occurs (It's so vague). Therefore Aero does not work. What could be wrong? What can we do to fix it?

Run the Vista Upgrade Advisor to figure out where the problem lies. Then, if possible, upgrade those components.

Let us know the exact error message and maybe we can give a bit more info.

The upgrade advisor does not mention any issues with the computer or video card. The Experience Index completes the analysis then says

"Cannot complete assessment. The assessment or other operation did not complete successfully. This is due to an error being reported from the operating system, driver, or other component."

You've installed the latest drivers for it?

With default Windows Drivers it produces 1.0. With ATI Catalyst Control Center installed. I cannot access the control center, and it produces the error. I have done two clean installs and got the same problem.