We have a problem with opening Office documents where some users get a login dialog and others don't. If the user press Cancel the dialog disappears and the document is opened properly. If the enter their credentials they are usually prompted the same dialog again (until they press Cancel).
All computers have the site in their Intranet-zone and they all have IE8. I believe the error is related to settings (registry?) on the computers as the same user can use another one without getting the dialog. The error exists mainly on machines running Windows 7. At least one tested on Windows 7 where it didn't work and then in Vista where it worked as expected (no dialog).
I have found a kb-article which I think can be related (http://support.microsoft.com/?id=943280) but as far as I understand I should have the error in Vista as well. I have been unable to test the solution (editing the registry).