The IInetLogInformation::GetClientUserName method obtains the name used by the client to log on.
virtual LPSTR STDMETHODCALLTYPE GetClientUserName(
IN PCHAR pszClientUserName,
IN PDWORD pcbSize
);
Logging modules should query for this method if a server instance has set the MD_EXTLOG_USERNAME flag of the MD_LOGEXT_FIELD_MASK metabase identifier.
LogExtFileUserName, MD_LOGEXT_FIELD_MASK