Hi Andrea,
Believe me, I understand what a nightmare a re-install might be, especially if you're not even sure it will fix the problem. I was in an easier position, because this was a brand new machine, so wiping the OS and starting over wasn't a big deal. I would have to spend an entire weekend building my DAW regardless. Was your machine working until recently? Did anything significant change (new hardware or software)?
One possible idea: you might try imaging your system drive (essentially, make a backup copy of it, and store it on an alternate drive or on a network location). Then, you can try formatting and re-installing the OS along with only VI and the eLicenser software to see if that even works. You'll need some 3rd party software to do this. Maybe someone here could recommend a brand, as I haven't done this myself.
Another possibility: I have my suspicions that somehow the C++ run-time libraries that are required for either Vienna or the eLicenser software became corrupted somehow. When I installed Sonar the first time, there was a run-time error during the installation process of these libraries. Afterwards, no matter how many times I uninstalled and re-installed the VI and eLicenser software, nothing worked. One thing I didn't try, however, was to uninstall those libraries and re-install the software (which presumably would re-install those if required). I'm a little hesitant to recommend this course of action, because I haven't tried it myself, and this is just speculation. Also, I have no idea what else are relying on those runtime libraries. So, I'd recommend imaging the system before doing this as well.
I'm not sure what else I could recommend. Vienna folks, it would be nice if VI came with some sort of diagnostic mode, or a separate little diagnostic app. That way, if something is going wrong, at least there's some sort of error log your programmers could look at. At the moment, the program just appears to fail silently instead of giving any sort of useful feedback.