Start with a series of MS Office extensions built in C++ as COM objects. Add a user who really badly wants to avoid the requirement of an installer with the necessary privileges to write to HKEY_CLASSES_ROOT.
I've seem bits of evidence that MS has created some sort of trick for shadowing HKEY_CLASSES_ROOT from HKCU.
Is this, in fact, possible? If so, can this be installed as a non-privileged MSI, or does it have to be arranged otherwise? And, finally, what's the minimum version of Windows required?