Busca "GetVersion", "GetVersionEx" o
ejecuta "winver"
Edito;
Código Delphi
[-]procedure TForm1.Button1Click(Sender: TObject);
var
Size: Cardinal;
hKey: Windows.HKEY;
Buffer: ShortString;
begin
Size := SizeOf(Buffer);
RegOpenKey(HKEY_LOCAL_MACHINE, 'software\microsoft\windows nt\currentversion', hKey);
RegQueryValueEx(hKey, 'productname', nil, nil, @Buffer, @Size);
RegCloseKey(hKey);
ShowMessage(string(PChar(@Buffer)));
end;