Hallo Leute, 
  
das Problem ist gelöst.  :)  
Der Übeltäter war der Virenscanner. Hier die Info: 
  
Windows 2003 problems after installation of SP1. 
There seems to be a compatibility issue with NOD32 v2.0 and some Windows 2003 SP1 installations.  
  
Solution: do not install / deactivate the IMON component.  
  
In general we do not recommend to install the IMON component on a server as it may interfere with other applications. IMON scans incoming POP3 and http traffic, so is suitable for workstation use. On a server (unless it's used as a workstation) this type of protection is not a necessity. As AMON, NOD32's on-access scanner works reliably under even a very heavy server load, you do not loose protection by not installing IMON on a server.