Here is an example how to detect the Windows version and set the default font name and size of your app if it is Windows Vista or higher.
Important: you have to enable the ParentFont property of each form in your project!

program MyApp;

uses
  Forms,
  SysUtils;

{$R *.res}

begin
  Application.Initialize;
  if CheckWin32Version(6) then
  begin
    Application.DefaultFont.Name := 'Segoe UI';
    Application.DefaultFont.Size := 9;
  end;
  Application.MainFormOnTaskbar := True;
  Application.CreateForm(TfrmMain_MyForm, frmMain_MyForm);
  Application.Run;
end.