Norton Anti Virus (ONLY Norton Anti Virus - even McAfee AV, also from Symantec, isn't doing it) is reporting the installer for 3.999z2 has a "heuristic threat" - in other words, not a real problem, just something that the code thinks map possibly be a problem, so blocks it on principle. It's been reported a few times and, if you have a working installation already, Pete has posted a dll only download over at his support forums at simFlight for NAV users to download.
BB686: It will not stop working until GlobalSign revoke the license AND your PC updates its list to show the certificate as invalid, which is usually done every few days. It could, however, happen within a few hours of the request to revoke the license, hence telling people that updating sooner rather than later is the best idea.