Update file used by Windows Update for updating programs and files in Windows Vista and Windows Server 2008; installed by the Windows Update Stand-alone Installer, which is run by Windows Update.

Each MSU file contains four elements:

1. Windows Update metadata that describes each update package
2. One or more .CAB files that contain the update data
3. An .XML file that describes the contents of the MSU file
4. A properties file read by Wusa.exe

Important: Previous versions of the Windows operating system used .MSI files for Windows updates.