Aaaah, now I understand. I didn't know the SuperUser site, will consider this next time. I actually have noticed in the Faq that ServerFault is for "professionals". I'm probably a border case: The PC I have the problems with is my corporate PC, the domain my company's domain, the workgroup my company's workgroup, I have to care about our network among thousand other things (we are a very small company), accidentally I have indeed the problem with my own PC, it is Saturday and I am sitting in my office...

Thank you! It worked, I could accept the answer of Tom2's question. BTW: Did I something wrong with my second question that is the reason for the downvotes? I had read the Faq before I asked to be sure if the question does belong here at all and checked for duplicate questions. I honestly had the feeling it's the right place and nobody wrote a comment what I did wrong.