If the bundled MFC library that comes with Lr, or the official redistributable package does not solve this, it is not Adobe's fault. The fault lies with the library loader on the OS. This will be clear the moment you try to run any other app that requires the same library at the same version.
Many shops refer to the problem of finding, maintaining and loading the right version of a library on Windows as "DLL Hell." Welcome to hell.
You could try running this as a real Administrator, via "Start -> Run...":
regsvr32 mfc100u.dll
It will give you a message telling you about success or failure. You might want to note what this message says.
This should have be done by the various installers already, and the service it calls should do the right thing under the covers that that the library is registered and versioned so user applications can find it.
The problem is that there are many, many places this can go wrong. In most cases, it is the windows registry that is at fault -- or you have malware on your computer that is keeping a corrupted copy of the library available. Do not rule out either of these problems.
I'd start by disabling all real-time anti-virus protection (this can really interfere with the installation of libraries), and checking the registry. Verify you have sound, malware-free system. I'd end by resinstalling the OS from scratch. Somewhere in-between you might find a solution.
