I had the same experience today as others have described: Outlook Express failed to initialize, and Task Manager showed msimn.exe hogging the CPU. The cause, as I had suspected, was a total system freeze-up yesterday (probably due to an unrelated incident) while Outlook Express was applying a newly created rule to move numerous messages from the Inbox to another folder. Inbox.dbx was therefore corrupted. After killing the task msimn.exe, I was able to restore Inbox.dbx from a backup. Although I lost three weeks of messages, some important, it was a lot better than losing everything. Outlook Express seems to be working normally once again.
Two lessons: (1) Make backups! (2) Windows has not yet outgrown our need for the reset switch.