- Make a folder in C Drive called Garmin (C:\Garmin)
- Copy a file (any file will do, size doesn't matter too) into this C:\Garmin folder
- rename the copied file to Mapsource.exe
- Run the Garmin installer
- Follow the onscreen instructions to install Mapsource
- Install a MfM map (note: Mapsource will not start unless there is at least one map installed)
This didn't work for me (Windows 7 x64). I made a registry key to 'pretend' there was previous version. After merging it into the registry, latest downloaded MapSource (from Garmin site) installed correctly.
Here is the key for 64-bit OS (not for 32 bit)
Windows Registry Editor Version 5.00
"InstallDir"="C:\\Program Files (x86)\\MapSource\\"
Copy this to a text file, rename the extension "reg". Then double click and merge to registry.