Determining the Current Version of WUA
Determine the version of Windows Update Agent (WUA) before you use it. The current version of WUA is determined by the version of the Wuaueng.dll that is running in the \System32 subdirectory of the current Windows installation. If the version of Wuaueng.dll is version 5.4.3790.1000 or a later version, WUA is installed. A version that is earlier than 5.4.3790.1000 indicates that Software Update Services (SUS) 1.0 is installed.
When a call is made to SUS 1.0 by using the WUA API, an HRESULT of WU_E_AU_LEGACYSERVER is returned.
You can also use the IWindowsUpdateAgentInfo::GetInfo method to retrieve the current file version of Wuapi.dll that is running on a computer. The IWindowsUpdateAgentInfo interface is not supported in WUA 1.0.
For more information, see Updating the Windows Update Agent.