It seems Ive found this happening after running games, watching video, or updating a video card driver.
Note: This link is only present where there is a problem going on with Aero.
We also show how to get to the troubleshooting tool at any time below.
Ive found its just faster to have it repair issues automatically.
Windows begins looking for what is causing the problem with the display.
We found this method to be easiest in Vista.
To do it, bang out services.msc into the search box on the Start menu.
After it restarts, transparency should be back and then close out of Services.
Check for Aero Problems AnytimeThe steps above are meant for troubleshooting no transparency specifically.
Then in the following screen press Display Aero desktop effects under Appearance and Personalization.
Give it a moment to come up, then follow through the wizard as shown above.
That is all there is too it.