怎么样防止程序二次运行？

[gwsr]

我的程序如下，防止程序二次运行，为什么不好使呢？请各位指教！
#include <vcl.h>
#pragma hdrstop
USERES("ELESand.res");
USEFORM("Main.cpp", PlatForm);
WINAPI WinMain(HINSTANCE, HINSTANCE, LPSTR, int)
{
HANDLE mutex ;
try
{
const char mutexname[] = "abcdedfg" ;
mutex = OpenMutex (0, false, mutexname) ;
if (mutex == NULL)
{
mutex = CreateMutex(NULL, false, mutexname) ;
}
else
{
ShowMessage ("application已经在运行！") ;
return 0 ;
}
Application->Initialize();
Application->CreateForm(__classid(TPlatForm), &PlatForm);
Application->Run();
}
catch (Exception &exception)
{
Application->ShowException(&exception);
}
ReleaseMutex (mutex) ;
return 0;
}