Outlook Express: password rejected

We have Outlook Express on 3 communal PCs with around 30 identities. For no apparent reason all 3 PCs suddenly have the same problem; all of the passwords are rejected when I try to switch identities meaning no-one can use their email.
If I setup one of the accounts again it works, but this would mean setting up 90 accounts with no guarentee the same thing won’t happen again.

any ideas?!

Win XP
Broadband connection sharing