For everybody thats need to limit the instance number of your application:

uses SysUtils, TLHelp32;
 
function AppRunningCount(const AExeName: string): integer;
var
  hSnap: THandle;
  p32: TProcessEntry32;
begin
  Result := 0;
  hSnap := CreateToolHelp32SnapShot(TH32CS_SNAPPROCESS, 0);
  p32.dwSize := Sizeof(TProcessEntry32);
  Process32First(hSnap, p32);
  repeat
    if SameText(AExeName, ExtractFileName(p32.szExeFile)) then
      inc(Result);
  until not Process32Next(hSnap, p32);
  CloseHandle(hSnap);
end;

Use:

// On DPR
// Limit to 1 instance
begin 
  if AppRunningCount(ExtractFileName(ParamStr(0))) = 1 then
  begin
  // Application codes
  end;
end.

Explaining more about this approach:
In a situation of multiples logons at the same machine the Mutex cannot provide a correctly singleton structure because the mutex handle still only under the local scope of the user. So the way to control the instance number is counting the number of times the process appear on the process list of the machine.

A light for everybody. First of all, I’ll use two other usefull functions I posted on this site:

And now the magic:

uses Windows, TLHelp32;
 
type
  TWalkOnHandlesCallback = function(AHwnd, AParentHWnd: HWND;
    AClass, AText: string; AFlag: Integer): boolean of object;
 
function WalkOnHandles(AParent: HWND; AFlag: integer;
  ACallback: TWalkOnHandlesCallback): Integer;
var
  CurrHWnd: HWND;
begin
  Result := 0;
  CurrHWnd := GetTopWindow(AParent);
  while CurrHWnd <> 0 do
  begin
    Result := Result + 1;
    if Assigned(ACallback) then
      if not ACallback(CurrHWnd, AParent,
        GetHwndClass(CurrHWnd), GetHwndText(CurrHWnd), AFlag) then
        Break;
    CurrHWnd := GetNextWindow(CurrHWnd, GW_HWNDNEXT);
  end;
end;
 
function WalkOnHandlesOfExe(AExeName: string; AFlag: integer;
  ACallback: TWalkOnHandlesCallback): Integer;
var
  PID, CheckPID: DWord;
  hSnapShot: THandle;
  ProcessEntry: TProcessEntry32;
  CurrHWnd: HWND;
begin
  Result := 0;
  PID := 0;
  hSnapShot := CreateToolhelp32Snapshot((TH32CS_SNAPALL), 0);
  ProcessEntry.dwSize := SizeOf(TProcessEntry32);
  Process32First(hSnapShot, ProcessEntry);
  repeat
    if ProcessEntry.szExeFile = AExeName then
    begin
      PID := ProcessEntry.th32ProcessID;
      Break;
    end;
  until Process32Next(hSnapShot, ProcessEntry) = false;
  CloseHandle(hSnapShot);
 
  if PID > 0 then
  begin
    CurrHWnd := GetTopWindow(GetDesktopWindow);
    while CurrHWnd <> 0 do
    begin
      Result := Result + 1;
      GetWindowThreadProcessID(CurrHWnd, @CheckPID);
      if CheckPID = PID then
        if Assigned(ACallback) then
          if not ACallback(CurrHWnd, 0,
            GetHwndClass(CurrHWnd), GetHwndText(CurrHWnd), AFlag) then
            Break;
      CurrHWnd := GetNextWindow(CurrHWnd, GW_HWNDNEXT);
    end;
  end;
end;

Now let me explain what this big code do.

This code its only a light, a simple idea, about how you can navigate through the Windows Objects (Handle) from everywhere.

A simple example:

  TForm1 = class(TForm)
    mmoLog: TMemo;
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    function WalkCall(AHwnd, AParentHWnd: HWND;
      AClass, AText: string; AFlag: Integer): boolean;
  end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  WalkOnHandlesOfExe('delphi32.exe', 0, WalkCall);
end;
 
function TForm1.WalkCall(AHwnd, AParentHWnd: HWND; AClass,
  AText: string; AFlag: Integer): boolean;
begin
  Result := True;
  mmoLog.Lines.Add(stringofChar(' ', AFlag)  + Format('%d: [ %s ] %s', [AHwnd, AClass, AText]));
  WalkOnHandles(AHwnd, AFlag + 2, WalkCall);
end;

This example will list all Windows Objects on the Delphi application and will result a idented log on the mmoLog.

Simple uhm?