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Particularly interested in Windows XP, but all versions of Windows would be useful (if they differ).

EDIT: How does Windows/Windows Update know what to apply and what it has already done? Does it compare .dll/.exe versions or something else?

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There is a log file located at %windir%\Windowsupdate.log which should have all information about updates performed on the system. Alternatively you could check the Add/Remove programs dialog and check off the Show Updates checkbox to see which ones are installed.

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However, WindowsUpdate.log can be deleted. So can the %WINDIR%\Software Distribution\ directory which contains WU downloads. Add/Remove Progs doesn't seem to contain all updates (for example, the Malicious Software Removal Tool). – Umber Ferrule Mar 2 '10 at 17:18

When you click on Windows Update in XP, you'll see a link to view your update history.

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Interesting - I never knew you could review this here. However, it doesn't necessarily list all WU. – Umber Ferrule Mar 2 '10 at 22:02
up vote 0 down vote accepted

I've just come across the following Microsoft web page which sheds some light on the question: Command-line switches for Windows software update packages

Under the heading, Overview of the installation process, it states:

  • The [hotfix] installation program registers the software update under one of the following registry keys:
    • HKEY_LOCAL_MACHINE\Software\Microsoft\Updates\<operating system>\<service pack level>\KB<nnnnnn>
    • HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Hotfix\Q<nnnnnn>
    • HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Hotfix\KB<nnnnnn>


  • Removal information for the software update is stored in a hidden folder named:
    • $NtUninstallQnnnnnn$ or $NtUninstallKBnnnnnn$ in %Systemroot%
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