Thank you Bravenue, that is correct. the entire 'fix' is;
You will need to manually remove the game folder, and delete 2 Registry Keys before you'll be able to re-install.
Go to C:\Program Files\Stardock Games\Sins of a Solar Empire (or where ever you installed the game) and delete the main folder.
Then launch the System Registry (Start-> run, and type regedit)
Look for and delete the following Keys;
For Vista 64 bit;
Then, reboot your PC and go back to Impulse and try installing the base game. Make sure it installs to 1.13 version, test it out before then installing entrenchment.