Since Avast isn't alerting (or you didn't mention it) it is strange that file shield wouldn't alert in some way.
Have you tried adding the Dexpot executable to the file system shield ?
I don't even recall if avast 10 has the behaviour shield or not, if so there is a possibility that could delay the start of Dexpot, causing it to fail. But your saying disabling the file system shield, see is display on the system tray.
I don't know how dexpot functions to do its desktop switching.
The real problem is your having avast 10, very few that have used this in the distant past and those that have, will have forgotten much of its functionality and or settings. Unfortunately I probably have more questions than answers.