Outlook Express keeps the messages from each email folder in a separate physical Windows file. If all your messages are in one folder, that file gets huge. Deleting an email leave a "hole" in the file but it doesn't actually shrink the file. Compacting a folder actually physically shuffles the emails within the file to eliminate all the "holes". If the file is too large, OE seems to get confused which locks it up.

I fixed one situation like this by creating several archive folders and moving groups of emails from the deleted folder and inbox to the archive folders. I think I started with several hundred megabytes in the deleted folder and created four or five archives, each with about 40mb.

Just to let you know, this wasn't on my computer! I have all of mine set to empty the deleted folders as soon as I exit the program. If I'm going to save a message, it goes into an archive. If I'm going to delete a message, I delete it!

edit to add:

I've done searches for methods to fix this situation. The solutions involved several techniques which amounted holding your tongue correctly while spinning three times. I suppose they might have worked if the computer I was working on actually had the problem they were addressing.

Also, the full blown version of Outlook stores all messages within one file, no matter how many email folders you have. It doesn't seem to have the same problems with compressing the messages.