Declare a global variable to your unit:
var atom : word;

on you mainforms OnCreate Event put this.....

procedure TForm1.OnCreate(sender : Tobject);
begin
   if GlobalFindAtom('PROGRAM_RUNNING) = 0 then begin  {Search the global
   atom table for the program thats running}
   atom := GlobalAddAtom('PROGRAM_RUNNING');    {add it to the table if its not found}
end
else
  begin
      MessageDlg('Program is already Running!',mtWarning,[mbOK],0);   {if its found, show a
      message}
      Halt;    {and halt the running of another instance of the program}
   end;
end;

 And on the programs on destroy event

 procedure TForm1.OnDestroy(Sender : TObject);
 begin
    GlobalDeleteAtom(atom);  {remove it from the global atom table}
 end;

 I have tried it and it works very well
 Regards Tim Carthew