I've heard of a similar problem before - the encryption software seems not to like that avast! is scanning the crypted drive container for some reason. The solution is to put the path to the contained into the list of Standard Shield exclusions (that prevent it from scanning it).
How to do that: leftclick on the avast! tray icon, click on "Details...>>" if not already expanded, select the Standard Shield provider from the list, click "Customize" and insert the path to the container on the last page of the provider configuration.
Since you didn't mention the name of the encryption software, I'm not sure if it's the same as the one I'm referring to; this was called "Protector Suite" and the path to the container was something like
c:\Documents and Settings\user\Application Data\Protector Suite\My Safe.fdp
(Of couse, you could have installed it elsewhere.)
In any case, the container file will be "big" - similar size as the encrypted partition - so it shouldn't be hard to find.