Why doesn't Windows Aero work on my Windows 7 system?
The setting for Windows Aero in Windows 7 is not applying on some systems. Certain Nvidia graphics cards, specifically the Nvidia GeForce 200 series cards, and some GeForce 9800 GT cards are affected. Revised drivers are expected over the coming weeks to correct this issue thought to be only a temporary condition.
Check the Nvidia Driver Downloads for updates when they are made available.