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

[Code]
const
  // our system-wide unique mutex name (at most MAX_PATH == 260 chars long,
  // case sensitive for comparison)
  MutexName = 'AAD45705-FD28-49E5-99A7-BB7638E16D3B';

function InitializeSetup: Boolean;
begin
  // do not continue if our mutex already exists (it means another instance
  // of this setup is running)
  Result := not CheckForMutexes(MutexName);

  // if our mutex was not found (no other instance is running), create one,
  // otherwise show a message and let the setup exit
  if Result then
    CreateMutex(MutexName)
  else
    MsgBox('Setup is already running.', mbInformation, MB_OK);
end;

function InitializeUninstall: Boolean;
begin
  // here if you create our mutex, the installer won't start (it prevents to
  // start the installer when an instance of the uninstaller is running)
  CreateMutex(MutexName);
end;