Update from what I initially thought to be a problem with the AVG Anti-Spyware, that seemed to also be replicated when I reverted to the Ewido incarnation. Now I find that the problem also happens after a scan with Spybot S&D and also after a scan with AdAware. It now seems that the problem is in fact with avast 4.7.892.

See http://forum.avast.com/index.php?topic=24033.0