You should go to the install directory and be sure everything is deleted.
And I think SimCity games save some files into your Documents folder, so you'll want to delete all that too.
Keep in mind, you'll lose every bit of data you had with the game - saves, maps, mods, settings, etc.
But if you can't reinstall, it's most likely because some old 'install data' is still on the computer, messing the installation up.