Windows Update: Does Windows record which updates are installed? If so, where?

Question

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?

Answer 1

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>

and

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

Answer 2

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.

Answer 3

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