When I tried to watch Youtube this evening I got a message on the screen which said:
"This page provides potential unsafe informationto an ActiveX control. Your security settings prohibit running control in this manner. As a result this page may not display correctly".
Trying to put an old school photo on the abomination that has become Friends Reunited, it told me to download Adobe Flashplayer. Then this trouble started. I've uninstalled it now.
Youtube requires Adobe Flash Player to be installed.
Check that 'Adobe Flash Player Plug-in' isn't still in your list of installed programs. (If it is, remove it and restart your computer). Then reinstall from here:
http://get.adobe.com/flashplayer/
Your ActiveX settings will be somewhere within your web browser's advanced settings but, since you've not told us which browser you're using, I can't tell you exactly where!