Peter is getting a popup on YouTube that is saying it has an expired certificate. Leo says that usually indicates an inaccurate date and time set in the computer. He should also update his browser. That will update the certificate authorities.
Another possibility is that the Heartbleed bug may have bit YouTube and they revoked all certificates as part of the fix. He'll have to flush his cache, reboot the computer and update his browser. He should do this anyway. There could also be a "man in the middle" attack going on with someone posing as YouTube. So he'll want to check that he's actually at youtube.com. But nine times out of ten, it's a clock error.