type
  TWinVersion = (wvUnknown, wv95, wv98, wvME, wvNT3, wvNT4, wvW2K, wvXP);

function DetectWinVersion: TWinVersion;
var
  OSVersionInfo: TOSVersionInfo;
begin
  Result := wvUnknown;
  OSVersionInfo.dwOSVersionInfoSize := sizeof(TOSVersionInfo);
  if GetVersionEx(OSVersionInfo) then
  begin
    case OSVersionInfo.DwMajorVersion of
      3: Result := wvNT3;
      4: case OSVersionInfo.DwMinorVersion of
          0: if OSVersionInfo.dwPlatformId = VER_PLATFORM_WIN32_NT then
              Result := wvNT4
            else
              Result := wv95;
          10: Result := wv98;
          90: Result := wvME;
        end;
      5: case OSVersionInfo.DwMinorVersion of
          0: Result := wvW2K;
          1: Result := wvXP;
        end;
    end;
  end;
end;

function DetectWinVersionStr: string;
const
  VersStr: array[TWinVersion] of string = (
    'Unknown',
    'Windows 95',
    'Windows 98',
    'Windows ME',
    'Windows NT 3',
    'Windows NT 4',
    'Windows 2000',
    'Windows XP');
begin
  Result := VersStr[DetectWinVersion];
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Ver := DetectWinVersion;
  Label1.Caption := IntToStr(Ord(DetectWinVersion));
  Label2.Caption := DetectWinVersionStr;
end;