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This happened to me yesterday, and I just want to rule out that anything malicious was going on.

I should probably also mention that it switched back to Aero after a minute or two.

edit: I have found the event in the log, IDs 9013 and 9010:

The Desktop Window Manager was unable to start because composition was disabled by a running application

A request to disable the Desktop Window Manager was made by process (Windows System Assessment Tool)

It was under Windows Logs -> Application in the Event Viewer.

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4  
This happens sometimes when your CPU just can't handle everything going on.. it's highly unlikely it is anything malicious, but I have no idea where a log that records where this happens. – cutrightjm Mar 25 '12 at 18:32
    
Related: superuser.com/questions/325714/… – Ƭᴇcʜιᴇ007 Mar 25 '12 at 19:05
2  
This can also happen with older software that is not Aero-aware. Once it closes, Aero can come back to life again. – user3463 Mar 25 '12 at 22:47
up vote 5 down vote accepted

I occasionally see this when my graphics driver crashes and restarts.

Check the Windows Event Viewer, particularly under Windows Logs, Application or Windows Logs, System. Look for anything with a yellow or red icon to indicate something went wrong.

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Thank you! So the culprit was "Windows System Assessment Tool" which disables Aero if the Windows Experience Index is below 3 ("Aero Assessment"). "WinSAT is scheduled to automatically run every week. The default schedule is 1am on Sundays." Good to know. en.wikipedia.org/wiki/Windows_System_Assessment_Tool – cptloop Apr 29 '12 at 23:11

This is probably WinSAT running. Check in Task Sheduler under Microsoft -> Windows -> Maintenance

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