Why has Matlab stopped recognizing the installed Compiler?

Everything was working and I was able to generate C code from a Simulink model just fine, but today, I get the following error:

An installed compiler was not detected. Certain simulation modes, as well as host-based coder builds require that a compiler be installed. Please install one of the supported compilers for this release as listed at:

http://www.mathworks.com/support/compilers/R2011a/win64.html

MATLAB must be restarted after the compiler is installed.

I am running 64 bit Windows 7 OS.

I am running Matlab R2011a (64 bit).

I have installed Visual C++ 2010 express.

I have installed Windows SDK 7.1

Inside Matlab, I have run 'mex -setup,' selected the Microsoft Visual C++ Compiler that it finds, and entered 'y' to save the information.

I have tried every possible combination of installing, uninstalling, re-installing Matlab, Visual C++ 2010 express, and the Windows SDK 7.1.

Why would mex -setup work for choosing the compiler but then fail to recognize that it has in fact been set up (especially given that it worked previously)?

[Matt J]

Inside Matlab, I have run 'mex -setup,' selected the Microsoft Visual C++ Compiler that it finds, and entered 'y' to save the information.

The error message told you to restart MATLAB after installing the compiler. Did you do that, too?