Login problem is solved. I just changed the authentication mode in the web.config file.
New Problem:
When doing final checkout (pressing 'Complete Order' button) I received the following message:
    Message queuing has not been installed on this computer!

After installing Message Queuing (with only the common subcomponent!?) i restarted my machine. Now, I receive another message saying:
    Queue is not registered in the DS.

Problem is that I do not have any experience with Message Queuing! I would appreciate a quick hint on how to solve this? If not I have to read all the long help texts on message Queuing. Life is cruel...
(Note: I'm running windows XP Professional on a Laptop machine)