system
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The problem occured for me because I had a patched version of explorer.exe. See here. If you had knowingly modified explorer.exe (with a program such as W7SBC) and trust that program, then you may do as I did and whitelist explorer.exe in Avast for now.
Of course, there is still the chance that explorer.exe is malware. So do it at your own risk.
linking all the threads together:
http://forum.avast.com/index.php?topic=147308
http://forum.avast.com/index.php?topic=147328
http://forum.avast.com/index.php?topic=147333 (this thread)
http://forum.avast.com/index.php?topic=147339
I have also alerted Avast of the file (although the last time I did this they took >3 months to reply…)