to be certain of the apparent unintended installation bug that I reported earlier, I re-installed version 6.03 over 6.1 [not quite the same thing as uninstalling 6.1, but I was in a bit of a hurry] -- both in my testing and production directories.
I started the installer and gave the target directory as c:\mirc\testing and again the installer froze after unpacking the last file (urls.ini) -- I then killed the process using Task Manager.
Then I re-ran the installer, specifying the same target testing directory as before, and this time the installer did NOT overwrite the production directory.
In other words, I was not able to reproduce the reported problem.
I could speculate on this, but the most likely candidates for clues to what happened [mirc.ini and the windows registry] have now been overwritten several times.
This does suggest an improvement for future versions of the installer: when the installer saves the current version files, it should also save mirc.ini.