Workaround #1: You might find that the problem was caused by installing SP1. The workaround is to uncheck the “use this identity as default” box under “Manage Identities”, and get prompted for which identity to use every time you start.
Workaround #2: Hit the [F1] key before switching identities. This will bring up the Outlook Express Help window. With this window minimised, you can switch identities.