Windows 7 - Aero Not Working Any More

One last thing to try, you might want to make sure that the performance options were not altered to have aero turned off.

to do so:
pull up the system properties -> click on Adv. system settings -> click on settings (under performance) -> make sure the check box labeled "adjust for best appearance" is checked.


PLAN B:

go into the device manager, check what your graphics card is detected as. If it is "generic Display Adapter" windows is not loading the driver(s). This could be for 1 of 3 reasons.

1. The most unlikely is that the GPU is not getting enough power. on a desktop this is a lot more possible
2. the driver is corrupted or not installed, try using a driver cleaner, something like driversweeper (use at your own risk)
3. the GPU could have stopped physically working for 3d applications. Under Warranty?