[Setup]
AppId=2336BF63-DF20-445F-AAE6-70FD7E2CE1CF
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Code]
#ifdef UNICODE
  #define AW "W"
#else
  #define AW "A"
#endif

const
  ERROR_ALREADY_EXISTS = 183;

function CreateMutex(lpMutexAttributes: LongWord; bInitialOwner: BOOL; lpName: string): THandle;
  external 'CreateMutex{#AW}@kernel32.dll stdcall';

function InitializeSetup: Boolean;
begin
  Result := True;
  // it's quite handy to use the AppId directive to be the mutex name because it should be unique;
  // if the CreateMutex function call fails, let the setup start no matter what happened; if this
  // call succeeded, and the mutex already exists, there is another process that created it - let
  // the setup exit telling this to the user
  if (CreateMutex(0, False, '{#SetupSetting('AppId')}') <> 0) and (DLLGetLastError = ERROR_ALREADY_EXISTS) then
  begin
    Result := False;
    MsgBox('Another instance of the setup with this AppId is running.', mbInformation, MB_OK);
  end;
end;