I just encountered the following error when installing SQL Server Upgrade Advisor on one of our client's production servers. This installation package could not be opened. Contact the application vendor to verify that this is a valid Windows Installer Package.
I tried several things such as restarting the Windows Installer service, registering Windows Installer, deleting the Windows Installer Package and downloading it again etc but nothing worked. Somehow, I had the feeling that this could be due to a failed network connection. So I downloaded the installer package on another machine(not on the same domain as the server), copied it to the server and ran it again and bingo, it installed successfully.
I hope this helps other folks who might run into the same issue.