Microsoft .NET Framework Error -- But I already installed the .NET Framework!!!!

If you have already installed the Microsoft .NET Framework (2.0, 3.5, whatever) on a client machine and try to run a program written using the framework that causes this type of error message:

Then do this:

First on the client's PC remove (through add/remove programs) ALL installations of the .NET Framework (1.1, 2.0, 3.0, 3.5, etc.) - remove them all. Reboot the machine after uninstalling all of the existing .NET versions.
Next, install .NET 3.5 SP1 from this site: http://www.microsoft.com/downloads/details.aspx?FamilyId=AB99342F-5D1A-413D-8319-81DA479AB0D7&displaylang=en
Once it has finished installing, reboot the machine.

For some reason when you install the .NET Framework, it doesn't REALLY install and just sits in Add/Remove Programs to taunt you into thinking it is installed.