Just an FYI, I was having the same problem mentioned above, every time I tried to install it it would show error writing to, and then list most if not every file involved (one at a time, if you hit Ignore on each of them). This happened even running the installer as an administrator (Right-click>Run as administrator)
Only way I found to do it was uninstall the old Mids, Delete the Mids folder out of Program Files, and then try again, after doing so, no problems at all.
I dunno if there is a better way to go about it, but whatever the easiest way to do it is, you might want to put a note about it on the CoH Planner page where you download Mids, just a quick little blurb like "If you have the old version of Mids you must..."
Thanks for the update!