When you apply a patch for an existing program that was installed with the Windows Installer, you may be prompted for the source files from the original installation. You are prompted to provide the source files from the original disk even if they are not needed for the actual patch.

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In most patching scenarios, the source will be necessary in order to reinstall necessary files. A patch uses the Windows Installer file versioning rules when deciding whether to reinstall a file contained in a feature that is being updated.

Being prompted for the source when no files are being reinstalled is due to a bug in the Windows Installer patching algorithm.

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When you perform the original installation from a network drive, Windows Installer can locate the files needed. However, the network location must be available when you apply the patch so that the source files can be found. Also, if you performed the original installation from a CD-ROM and the disc is still available, insert the CD-ROM disc when prompted.

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Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article.

The unnecessary prompting for the original installation source when you apply a Windows Installer patch is improved in Windows Installer version 2.0. In Windows Installer 2.0, you may be prompted for the original installation source when applying a patch.

Any of the following conditions can cause a prompt for the original installation source:

•	If the feature to be patched has components with unversioned files that are not modified on the computer, Windows Installer requires the source unless the unversioned files are authored in the appropriate MsiFileHash table entries in the original setup package.
•	When you call the ResolveSource Windows Installer standard action from a custom action during the patch application requires the original source.
•	When you call the ResolveSource Windows Installer standard action in the original product setup during the patch application requires the original source.
•	If the feature to be patched is currently set to Run-From-Source. Setting a feature to run from source implies that the feature does not have to be installed locally and should run from the original installation source.
•	The cached .msi file for the product that is being patched is missing from the machine. This can happen if the files under the computer %windir%\installer directory are erroneously deleted by an end user or software program.

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Windows Installer 2.0 Redistributable for Windows NT 4.0 and Windows 2000
http://www.microsoft.com/downloads/details.aspx?familyid=4B6140F9-2D36-4977-8FA1-6F8A0F5DCA8F (http://www.microsoft.com/downloads/details.aspx?familyid=4B6140F9-2D36-4977-8FA1-6F8A0F5DCA8F)

Windows Installer 2.0 Redistributable for Windows 95, Windows 98, and Windows Millenniium Edition
http://www.microsoft.com/downloads/details.aspx?FamilyID=cebbacd8-c094-4255-b702-de3bb768148f%20&displaylang=en (http://www.microsoft.com/downloads/details.aspx?FamilyID=cebbacd8-c094-4255-b702-de3bb768148f%20&displaylang=en)

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