Hello and good morning ( my time ),
Adobe flash player is PERFECTLY fine. If you were visiting another website, and it just came up and asked you to install flash player, then i would say you have spyware and its trying to install more crap into your PC using a legitimate software name. BUT, in your case, you're on Youtube.com, and youtube needs flash player, and if you don't have it, you need to get it because as you realized, most webpages use flash players. Now, it just leaves me wondering what antivirus you have that it doesn't recognize Adobe Flash Player. I could bet a dollar its Norton, but hey, i could be wrong. Try ZoneAlarm anti virus internet suite or the Extreme version. If you want the program to learn the way your computer runs, the first week it will ask you for your permission for everything, and it may bother some people clicking yes or no and to remember the setting for next time. But, if you want to skip that part, it almost becomes a REAL WALL between your computer and the internet.