The Security Mentor

Youtube videos, and a lot of those annoying flashing ads, come to you courtesy of a third-party plugin ("Flash") in your browser. Sometimes it has security vulnerabilities that let the files it shows take over your computer. It's got one now, and last I heard there was no patch available. Meantime bad guys are taking over legitimate web sites and using them to send you hostile Flash files.

You're fairly well protected if you're a Firefox user and have the NoScript extension installed.

If you run Internet Explorer, you can either temporarily disable or uninstall Flash. I recommend uninstalling it and then, if you want, reinstalling it later after there's a fix for the current problem. Here are instructions for uninstalling the Flash plugin. Youtube, a number of games, and a lot of annoying ads will stop working until you reinstall.