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Just now it happened, and after a couple minutes it turned back on, which is not something that happened before. Before this I have only seen it turning off by itself and I had to restart it after a few days to restore it myself.

I am not running any application that doesn't support aero, so that shouldn't be it. My PC also doesn't use power-saving settings either.

Why would windows 7 do this?

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maybe the service or the GPU driver crashed. Look into the eventlog if you have errors. –  magicandre1981 Dec 30 '12 at 6:16
    
Thanks, do you know how to access that? Is it the win7 generic eventlog? It didn't show any tooltip in the tray for this but sometimes I used to get "driver crashed and so the application" tooltip in the tray. –  Joan Venge Dec 30 '12 at 6:22
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yes, look into the Windows Eventlog: sevenforums.com/tutorials/…. –  magicandre1981 Dec 30 '12 at 6:23

2 Answers 2

It might by caused by the Windows System Assessment Tool, which Windows 7 runs every Sunday at 1 AM by default. Aero is disabled while it's running.

More info: http://en.wikipedia.org/wiki/Windows_System_Assessment_Tool

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Are you sure it runs every sunday? I don't know why would it do this since hardware changes don't happen every week. –  Joan Venge Dec 31 '12 at 1:00
    
this is indeed a good idea. Win7 has a scheduled tasks which runs when the PC is idle for some times. This task also generates the Index. I always disable it, so I forgot this: troublefixers.com/… –  magicandre1981 Dec 31 '12 at 6:32
    
Yes, at least on my system it does happen at that time, the Wikipedia article also mentions it. Ending WinSAT.exe in the task manager or waiting for it to finish running restores Aero. –  purkki Dec 31 '12 at 11:11
    
Right, that's what it was just now! –  user1306322 Nov 30 '13 at 21:03

Aero is highly dependent of your graphics card. if the driver fails as you mentioned, Aero is prone to get disabled. This driver failure may be caused by a faulty installation of your graphics drivers or them being outdated. Make sure you have up-to-date drivers, provided from the manufacturer.

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