Windows Update Standalone installer is a small utility that is used to install standalone updates. Standalone updates are updates that are not automatically available at Windows Update in your Windows PC. These are special types of updates that are used or generated for a specific group of users; like the Windows6.1-KB977206-x86.msu update required to install Windows XP Mode on PC’s which do not support Hardware Virtualization.

Windows Update Standalone Installer

The Windows Update Stand-alone installer viz. wusa.exe was introduced with some new features and improvements.

The Windows Update Stand-alone Installer (Wusa.exe) provides the following improvements in Windows 10/8/7:

Uninstall support: Prior to Windows 7, wusa.exe included install support only. In Windows 10/8/7 wusa.exe includes uninstall support so that administrators can uninstall updates from a command line. Users can uninstall an update by providing the path to the .msu file or by providing the package number (from the Microsoft Knowledge Base) of the update to be uninstalled.

1. Use the following command to uninstall an update by specifying the full path to the update:

wusa.exe /uninstall <Path>

2. Use the following command to uninstall an update by specifying the update package number from the Microsoft Knowledge Base:

wusa.exe /uninstall /kb:<KB Number>

Additional command-line parameters: New parameters are available in Windows 7 to enable logging, extract the contents of an .msu file, and control the restart behavior when an update is installed in quiet mode./log: This Switch is not available in earlier versions, Logging could only be enabled through tracing tools. This new parameter enables logging through the Wusa.exe tool./extract: Previously contents of .msu files could only be extracted by using the expand.exe tool. This new parameter enables .msu files to be extracted through the Wusa.exe tool./quiet: Supports the /norestart option only. Now The Switch functionality is extended to support the /forcerestart, /warnrestart, and /promptrestart options.
Extended error information. The Wusa.exe tool provides extended information in error scenarios for better diagnosis. [1] Update is already installed:1 (S_FALSE)0x240006 (WU_S_ALREADY_INSTALLED) [2] Update is not applicable:1 (S_FALSE)0x80240017 (WU_E_NOT_APPLICABLE)
Note: An update was released to provide the error codes for Windows 7 on computers that are running Windows Vista. For more information about the update, see KB949545.

Additional read at KB934307.