When a package is installed, Windows caches the MSI by
placing it in the Windows\\installer folder.
During this process, all the unnecessary information (including
the digital signature) is removed in order to decrease the size of
the file. When an uninstall is launched from "Add or Remove
programs" or through the Uninstall shortcut, Windows Installer
uses the cached MSI. Since this file doesn't have a digital
signature, the "Unknown Publisher" message will be shown. A
solution for this is to make sure the user can uninstall the
package only by launching the original file.