There are few possible solutions to this issue. First, ensure your card does indeed support the minimum requirements of Windows Aero:

DirectX 9 or higher Pixel Shader 2.0 or higher 128MB or greater video memory

If your video card does support these requirements, try the following:

1. Download and perform a clean install of an older driver (during the driver install, chose "Custom" then check the box on the next page that says "Perform a clean install). Once done, right click your desktop, chose Personalization. Change your theme to a different theme, then back to the Windows theme. This should re-enable Aero features.

2. If that fails, close any programs you have that are running Java, then restart your system.

3. Open Run or Search, and type services.msc then hit enter to open the Windows Services Manager. Find the "Desktop Windows Manager" service and make sure it's set to Automatic.

4. Open a Command Prompt, and copy the following command into the Command Prompt then hit enter:

rundll32.exe Dwmapi.dll,DwmEnableComposition

5. If all of these fail, your Windows install may be corrupt. The only recourse at this point would be to reinstall Windows.