To be honest, what you described sounds less like Norton misbehaving on its own and more like you having a virus on the computer which is causing Norton to flip out, as many viruses do.
Megaupload is an unsafe site in the grand scheme of things, and porn is probably the single most unsafe data entity you can ever download to a computer. I'm not saying that all porn has viruses hiding inside of it! (lol) What I am saying, though, is that you're much more likely to download a virus hidden inside of a porn video than you are to download a virus hidden inside of anything else.
From the perspective of a man or woman searching for porn, the problem with sites like RapidShare and Megaupload is that they are too well-known. Four years ago, barely anybody knew of or used Rapidshare outside of the porn and hentai communities. Two years ago, people were using it for anything mainstream otaku. Today, people are using it for anything mainstream period. One to two weeks ago, even, I saw a middle-aged man who is computer incompetent going on Rapidshare to download some academic files he had uploaded to it earlier in the week. When a file-sharing service has that level of publicity, it's bad news for everybody. Big publicity means bigger incentive to upload an infected file on that server and infect a greater number of computers before the file is taken down. If a virus is attached to this forum, five to ten people might get infected before the attachment would be deleted. But how long does it take for Megaupload to take down an infected file? and in that time window, how many people do you think download it?
I know I'm preaching on somewhat of an off-topic, but I really do think that you should be less concerned with which of the top five anti-virus softwares you're running and be more concerned with your websurfacing practices. Look at it this way: if a guy buys the best condom on the market, he's still at more risk for getting an STD than the guy who buys the average condom if the first guy is having sex down at the pier with every hooker he can find whereas the second guy is having monogamous sex with his girlfriend. Where you go on the Internet and what you download from it plays a much bigger role in security compromisation than the flavor of your anti-virus software.