I've been having this problem for about a month.
I don't have any gadgets. In fact, they are disabled on Windows Resources under Control Panel.
I do have IE9, though. But the problem started appearing well after I installed it.
I'm suspecting some Update is the actual culprit.
EDIT:
Did some testing. In my case, the problem was with IntelliPoint version 8.15. (It was served to me via Windows Update, so it technically was an update =P).
The new version added support for the new Touch Mouse, which includes the new Instant Viewer (Mac Expose) for Windows 7, which is probably messing up with the Aero Peek. =\
If someone can send this to Microsoft.