Apparently Silverlight cannot count to 10 . Or is it that Cocoa can't count to 10 ? Anyway you can workaround the issue of not being to install Silverlight on Mac OS X 10.4.10 by doing the following: Goto the Silverlight package in the Finder Right click on the package, and "Show Package Contents" Navigate down to Contents->Resources Rename InstallationCheck to InstallactionCheckA (or whatever) This will make the installer to skip the Installation Check, and allow you install on Mac OS X 10.4.10.