Outlook Express: repeatedly unable to compact message

I have what appears to me to be a very unusual problem.

When I try to close Outlook Express I invariably get the following message:

To free up disk space, Outlook Express can compact messages. This may take up to a few minutes.

Several times I have clicked OK to compact my messages, and, invariably, I get the same result:

The folder is currently in use by Outlook Express or another application.

It seems very strange that I should continue to receive this message, especially since it appears even when I don't have any programs open. Also, why am I repeatedly given the option to compact my messages, only to have the compacting process fail every single time? This seems very odd to me. There must be something unusual occurring in the background, something of which I am unaware.

I would appreciate hearing from someone who may have an idea as to what exactly is happening with Outlook Express.