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  if FileExists(HelpPfad + HelpFileName) then
    ShowURL('file:///' + HelpPfad + HelpFileName, '#' + Dummy_String);

procedure TForm1.ShowURL(URL, Nav: string);
begin

  if Length(Trim(Nav)) > 0 then
    URL := '"' + URL + Nav + '"';
  ShellExecute(Application.Handle, PAnsiChar('open'), PAnsiChar(Url), PAnsiChar(Nav), nil, SW_SHOWNORMAL);

end;                    

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procedure TForm1.Button2Click(Sender: TObject);
var
  Befehl      : string;
  StartupInfo : TStartupInfo;
  ProcessInfo : TProcessInformation;
begin
  FillChar(StartupInfo, SizeOf(TStartupInfo), 0);
  StartupInfo.cb := SizeOf(TStartupInfo);
  Befehl := '"c:\program files\internet explorer\iexplore.exe" File:///f:\test2\a_name.htm#kap02';
  CreateProcess(nil,
                PChar(Befehl),
                nil,
                nil,
                false,
                0,
                nil,
                'c:\',
                StartupInfo,
                ProcessInfo);
end;                    

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function TForm1.GetDefaultBrowserPath: String;
var Reg: TRegistry;
    DefBrowser: String;
begin
  result := '';
  Reg := TRegistry.Create;
  Reg.RootKey := HKEY_CLASSES_ROOT;
  if Reg.OpenKey('.html', false)
  then
  begin
    DefBrowser := Reg.ReadString('');
    Reg.CloseKey;
    if Reg.OpenKey(DefBrowser + '\shell\open\command', false)
    then
    begin
      result := Reg.ReadString('');
      Reg.CloseKey;
    end;
  end;
  Reg.Free;
end;                    

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Windows Vista+
HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\Associations\UrlAssociations\http\UserChoice -> ProgID
 --> HKEY_CLASSES_ROOT\IE.HTTP\shell\open\command -> Standard IE
 --> HKEY_CLASSES_ROOT\FirefoxURL\shell\open\command -> Standard FF

Windows 9x+
 HKEY_CLASSES_ROOT\http\shell\open\command                    

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function TForm1.GetDefaultBrowser(var _path, _browser: string): Boolean;
var
  Reg: TRegistry;
  Dummy_String: string;
  Dummy_Bool: Boolean;
  DefBrowser: string;
begin

{
  HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\Associations\UrlAssociations\http\UserChoice - > ProgID
    - - > HKEY_CLASSES_ROOT\IE.HTTP\shell\open\command - > Standard IE
    - - > HKEY_CLASSES_ROOT\FirefoxURL\shell\open\command - > Standard FF

  HKEY_CLASSES_ROOT\http\shell\open\command
}

  Result := False;
  Reg := TRegistry.Create;

  Reg.RootKey := HKEY_CURRENT_USER;
  Dummy_Bool := Reg.OpenKeyReadOnly('HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\Associations\UrlAssociations\http\UserChoice');
  if Dummy_Bool then
  begin
    Dummy_String := Reg.ReadString('ProgID');
    Reg.CloseKey;

    Reg.RootKey := HKEY_CLASSES_ROOT;
    Dummy_Bool := Reg.OpenKeyReadOnly(Dummy_String + '\shell\open\command');
    if Dummy_Bool then
    begin
      DefBrowser := Reg.ReadString('');
      Reg.CloseKey;
    end;
  end;
  Reg.CloseKey;

  if not Dummy_Bool then
  begin
    Reg.RootKey := HKEY_CLASSES_ROOT;
    Dummy_Bool := Reg.OpenKeyReadOnly('.html');
    if Dummy_Bool then
    begin
      DefBrowser := Reg.ReadString('');
      Reg.CloseKey;
      Dummy_Bool := Reg.OpenKeyReadOnly(DefBrowser + '\shell\open\command');
      if Dummy_Bool then
        DefBrowser := Reg.ReadString('');
      Reg.CloseKey;
    end;
  end;

  Reg.Free;

  if Dummy_Bool then
  begin
    Dummy_Bool := Length(DefBrowser) > 0;
    while Pos('""', DefBrowser) > 0 do
      DefBrowser := StringReplace(DefBrowser, '""', '"', [rfReplaceAll]);
    if Pos('"', DefBrowser) > 0 then
    begin
      if DefBrowser[1] = '"' then
        DefBrowser := copy(DefBrowser, 2, MaxInt);
      DefBrowser := copy(DefBrowser, 0, Pos('"', DefBrowser) - 1);
    end;
    while Pos('"', DefBrowser) > 0 do
      DefBrowser := StringReplace(DefBrowser, '"', '', [rfReplaceAll]);
    _path := ExtractFilepath(DefBrowser);
    _browser := ExtractFilename(DefBrowser);

    Result := Dummy_Bool;
  end;

end;

procedure TForm1.ShowURL(URL, Nav: string);
var
  BrowserPfad, BrowserBin: string;
  Befehl: string;
  StartupInfo: TStartupInfo;
  ProcessInfo: TProcessInformation;
  Dummy_Bool: Boolean;
begin

  if Length(Trim(Nav)) > 0 then
    URL := URL + Nav;

  Dummy_Bool := GetDefaultBrowser(BrowserPfad, BrowserBin);

  if Dummy_Bool then
  begin
    FillChar(StartupInfo, SizeOf(TStartupInfo), 0);
    StartupInfo.cb := SizeOf(TStartupInfo);
    Befehl := Format('"%s" "%s"', [BrowserPfad + BrowserBin, URL]);
    CreateProcess(nil,
      PChar(Befehl),
      nil,
      nil,
      false,
      0,
      nil,
      'c:\',
      StartupInfo,
      ProcessInfo);
  end;

  if not Dummy_Bool then
    ShellExecute(Handle, PAnsiChar('open'), PAnsiChar(Url), nil, nil, SW_SHOWNORMAL);

end;