Run the mIRC beta installer. Uncheck everything except "Program File" when asked what you want to install. At the next screen, uncheck the first three boxes (creating shortcuts and backing up old files) and check the last box (run as portable application).
This will give you exactly what you want: The only files that will be replaced will be mirc.exe, uninstall.exe, and the defaults folder. Checking "run as portable application" is really just preventing it from making changes to the registry. Since it isn't overwriting your ini file or shortcuts, it will run the same as it did before the upgrade process.
You say you do not want the "installed versions of mIRC", but that doesn't really make sense. The only thing the installer does is create shortcuts and things of that nature, and you can prevent them from happening using the above choices.
