"This installation package could not be opened."

I see a lot of threads for the above message but I haven't found a
resolution. I can't believe editing the registry is the only way to
solve this issue--a Microsoft guy came up with that one and it didn't
work for the poster.

This happened with iTunes and it happened ith the latest version of
Windows Installer.

Worst of all, it seems like the people with this problem just fall off
the earth....