Came across this error today whilst trying to install Silverlight for an enterprise LOB app, and as you know if you come across this error the support documentation on MSDN is pretty useless. The machine had the default Silverlight v3 install and we were trying to install an instance of v4. So first thoughts was local admin rights - I didn't have them, after finding someone who did, it was still failing. Next attempt was following the steps defined here but even after uninstall\reinstall still no luck. I started to think this was going to be a long afternoon. Then I remember a great little tool which is no longer available for download from MSDN -' Windows Installer Clean Up ', great little tool for cleaning the registry and file system of installed applications, but in the wrong hands this can totally wreak a machine, the reason why MS pulled it from MSDN, article here . Fortunately for me there's a copy on the corporate network, it's got a rather strange file...