The online installer is signed using sha256 which isn't supported on Windows XP SP2. You can try the standalone installer posted at the bottom of the Installation problems | Flash Player | Windows 7 and earlier page in the 'Still having problems' section, however, there is no guarantee that it'll work. Your best bet is to upgrade your system, if possible, to a newer supported operating system version, or install Service Pack 3. This page, ssl - SHA2 support for windows XP SP2 : any hotfix or dll available? - Super User, may be of assistance.