Check whether other machines have the same problem. If no - try to reinstall the affected machine (yes I know it is drastic step, but it can be a broken dll or service misconfiguration and the time you will loose identifying problem may overcome the time to reinstall everything)

>I have no zero administration. Norton Antivirus is installed. I have the same problem even when the antivirus is disabled...
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>>I have tryed to reproduce it with no success. My OS is Win2k Pro Build 2195 SP3.