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;