I opened a video on YouTube.com, and now I keep getting their logo in several pages, is it a virus?

Answer 1

Likely, you now have a cookie from youtube on your computer and when you go to certain sites it is reading that cookie and changing the content (the ads) based on that cookie. It is a virus... no. It is 'Adware', which can be almost as bad. In this case, it sounds pretty harmless.

If it really bothers you, get an 'adware' cleaning utility or just delete your cookies (from the Tool/Internet Option menu option) and it should go away.

That is a guess based on the information you gave.