private
  { Private declarations }
  procedure OSInfo;
...

procedure TForm1.OSInfo;
var
  BRes : boolean;
  lpVersionInformation : TOSVersionInfo;
  c : string;
begin
  lpVersionInformation.dwOSVersionInfoSize :=
  SizeOf(TOSVersionInfo);
  BRes := GetVersionEx(lpVersionInformation);
  if BRes then
    with lpVersionInformation do
      case dwPlatformId of
        VER_PLATFORM_WIN32_WINDOWS :
          if dwMinorVersion=0 then
            c := 'Windows 95'
          else
            c := 'Windows 98';
        VER_PLATFORM_WIN32_NT :
          c := 'Windows NT';
        VER_PLATFORM_WIN32s :
          c := 'Win 3.1 with Win32s'
      end;
  Form1.Caption:=c;
end;