In Inno Setup, figure out if we're running an upgrade or a fresh install

gistfile1.txt

// Used to determine if we are running a new install or an upgrade by checking
// for the AppID in the registry.
function IsNewInstall(): Boolean;
var
  AppID: String;
  UninstallPath: String;
begin
  AppID := '{enter-AppID-here}'
  UninstallPath := 'Software\Microsoft\Windows\CurrentVersion\Uninstall\' + AppID + '_is1';
  Result := not (RegKeyExists(HKEY_CURRENT_USER, UninstallPath) 
    OR RegKeyExists(HKEY_LOCAL_MACHINE, UninstallPath));
end;