[Problem]
After installing Windows XP Professional (lol..) on a box, and running windows update (283 updates available. Do you want to download the internet?), i recognized some weird change in the user behaviour. Instead of connecting to windows servers using the username / password you logged in with, the connections from this client always showed up as "Guest". Huh?
[Solution]
After searching for quite a while, i found an entry in the Local Security Policies, called "Network Access: Sharing and security model for local accounts". Appearently, this is nowadays per default set to the "Guest" option - no matter what user you logged in with, the box will always authenticate as guest user to any other share you connect to. Change this to the "classic" mode... and have fun with your working shares, user-, and groupcontrol.
... oh my. Please, god, kill me - now.