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.