HANDLE m_hMutex=CreateMutex(NULL,TRUE, "YourApp");
if(GetLastError()==ERROR_ALREADY_EXISTS)
{
char szError[60];
LoadString(AfxGetInstanceHandle(),IDS_ERROR_PRGMRUNNING,szError,60);
MessageBox(NULL,"The program is running!","MyApp",MB_OK|MB_ICONINFORMATION);
return FALSE;
}