function GetOSVersion(var MajorVersion, MinorVersion, Build: DWORD) : String;
var
  VersionInfo: TOSVersionInfo;
  vPlatform: string;
begin
  VersionInfo.dwOSVersionInfoSize := SizeOf(VersionInfo);
  GetVersionEx(VersionInfo);
 
  with VersionInfo do
  begin
    case dwPlatformId of
      VER_PLATFORM_WIN32s:
        vPlatform := 'Windows 3x';
      VER_PLATFORM_WIN32_WINDOWS:
        vPlatform := 'Windows 95';
      VER_PLATFORM_WIN32_NT:
        vPlatform := 'Windows NT'; // 2000, XP, Vista, 7, 8, 8.1
    end;
 
    MajorVersion := dwMajorVersion;
    MinorVersion := dwMinorVersion;
    Build := dwBuildNumber;
 
    Result := vPlatform;
  end;
end;

procedure TForm1.Button2Click(Sender: TObject);
var
  MajorVersion, MinorVersion, Build: DWORD;
begin
  ShowMessage(GetOSVersion(MajorVersion, MinorVersion, Build));
 
  if ((MajorVersion = 5) and (MinorVersion = 0)) then
    ShowMessage('Windows 2000 Build: ' + IntToStr(Build));
 
  if ((MajorVersion = 5) and (MinorVersion = 1)) then
    ShowMessage('Windows XP Build: ' + IntToStr(Build));
 
  if ((MajorVersion = 6) and (MinorVersion = 0)) then
    ShowMessage('Windows Vista Build: ' + IntToStr(Build));
 
  if ((MajorVersion = 6) and (MinorVersion = 1)) then
    ShowMessage('Windows 7 Build: ' + IntToStr(Build));
 
  if ((MajorVersion = 6) and (MinorVersion = 2)) then
    ShowMessage('Windows 8 Build: ' + IntToStr(Build));
end;