You can't get a virus from watching a video posted on the official YouTube website. If that's the case, the warning is probably triggered by one of the ads that are being displayed, so if you don't click on any of them you are completely safe.
However, if you received the link to the video via email, chat application or if you found it on some website, read the URL address very carefully and make sure that it's actually an YouTube address and not something very similar meant to trick you into visiting a malware infested website. Misspelled YouTube links are commonly used in phishing schemes, so pay close attention to the address: make sure that it starts with https (not just http) and that it's really YouTube that you're visiting.
Lastly, are you sure that the content warning came from Norton Security and not from YouTube itself? Google's video sharing service usually warns you about the content when you try to watch a video that other users have flagged as adult or offensive and it has nothing to do with PC security just with what you're going to see in the video.