This error occurs because the installer cannot install the upgrade patch because the program being upgraded may be missing, or the upgrade patch updates a different version of the program.
There is a solution when ERROR_PATCH_TARGET_NOT_FOUND
Verify that the program to be upgraded exists on your computer and that you have the correct upgrade patch. If not, obtain the latest service pack for the Microsoft .NET Framework 1.1. More information can be found on the service pack at this web address: http://support.microsoft.com/kb/885055/
TipFor more information, please see: How to fix Update error #1642