Here is an example that should work under all versions of Windows.

{$IFDEF WIN32}
function GetVersionEx(lpOs : pointer) : BOOL; stdcall;
 external 'kernel32' name 'GetVersionExA';
{$ENDIF}
procedure GetWindowsVersion(var Major : integer;
                            var Minor : integer);
var
 {$IFDEF WIN32}
  lpOS, lpOS2 : POsVersionInfo;
 {$ELSE}
  l : longint;
 {$ENDIF}
begin
 {$IFDEF WIN32}
   GetMem(lpOS, SizeOf(TOsVersionInfo));
   lpOs^.dwOSVersionInfoSize := SizeOf(TOsVersionInfo);
   while getVersionEx(lpOS) = false do begin
     GetMem(lpos2, lpos^.dwOSVersionInfoSize +  );
     lpOs2^.dwOSVersionInfoSize := lpOs^.dwOSVersionInfoSize +  ;
     FreeMem(lpOs, lpOs^.dwOSVersionInfoSize);
     lpOS := lpOs2;
   end;
   Major := lpOs^.dwMajorVersion;
   Minor := lpOs^.dwMinorVersion;
   FreeMem(lpOs, lpOs^.dwOSVersionInfoSize);
 {$ELSE}
  l := GetVersion;
  Major := LoByte(LoWord(l));
  Minor := HiByte(LoWord(l));
 {$ENDIF}
end;
procedure TForm .Button Click(Sender: TObject);
var
  Major : integer;
  Minor : integer;
begin
  GetWindowsVersion(Major, Minor);
  Memo .Lines.Add(IntToStr(Major));
  Memo .Lines.Add(IntToStr(Minor));
end;