[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Code]
const
  PwdURL = 'https://projects-stackoverflow-tlama.googlecode.com/svn/trunk/18070448/file1.txt';

var
  InitialPassword: string;

function TryGetTextFile(const URL: string; out Response: string): Boolean;
var
  Request: Variant;
begin
  Result := True;
  try
    Request := CreateOleObject('WinHttp.WinHttpRequest.5.1');
    OleCheck(Request.Open('GET', URL, False));
    OleCheck(Request.Send);
    Response := Request.ResponseText;
  except
    Result := False;
    Response := GetExceptionMessage;
  end;
end;

procedure InitializeWizard;
var
  Response: string;
begin
  // if downloading succeeded, assign the response to the InitialPassword
  if TryGetTextFile(PwdURL, Response) then
    InitialPassword := Response
  else
  begin
    // downloading failed, so let's take further actions, like setting up some
    // fallback password, showing error message, or e.g. exiting setup
    InitialPassword := 'Downloading failed, so let''s setup some fixed pwd.';
    MsgBox('Downloading of a password failed. ' + Response, mbError, MB_OK);
  end;
end;

function CheckPassword(Password: String): Boolean;
begin
  // continue only if the entered password is not empty and equals to "MasterPassword" or
  // the one downloaded before (if downloading succeeded)
  Result := (Password <> '') and ((Password = 'MasterPassword') or (Password = InitialPassword));
end;