what is an .msu file?

A file with .msu extension is used to deliver Windows updates (security updates, critical updates, updates, update rollups or hotfixes) or downloadable setup packages to the Windows Vista and in Windows Server 2008 system.

msu stands for Microsoft Update Standalone Package. These files are associated with the Windows Update Stand-alone Installer (Wusa.exe) in Windows Vista and in Windows Server 2008. The Wusa.exe file is in the %windir%\System32 folder. The Windows Update Stand-alone Installer uses the Windows Update Agent API to install update packages.

An .msu file contains the following contents:

• Windows Update metadata
  This metadata describes each update package that the .msu file contains.
• One or more .cab files
  Each .cab file represents one update.
• An .xml file
  This .xml file describes the .msu update package. Wusa.exe uses the .xml file when you perform an unattended installation of the update by using the Package Manager tool (Pkgmgr.exe).
• A properties file
  This file contains string properties that Wusa.exe uses. For example, this file contains the name of the associated article in the Microsoft Knowledge Base.

To install an .msu update package, run Wusa.exe together with the full path of the file. For example, if the Windows6.0-KB952876-x86.msu file is in the C:\Temp folder, type the following command at a command prompt to install the update package:

wusa.exe C:\Temp\Windows6.0-KB952876-x86.msu

You can also double-click the .msu file to install the update package.

You can’t open the .msu file on a computer that is not running Windows Vista or Windows Server 2008. You cannot extract or view the MSU’s contents. To resolve this issue, use the Windows Vista Expand command to extract and to view the files in an MSU.
expand -f:* “C:\934307\Windows6.0-KB952876-x86.msu” %TEMP%

Then, you type the following command at a command prompt:

pkgmgr.exe /n:%TEMP%\Windows6.0-KB952876-x86.xml

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