Hi
(1) Test your Flash and Javascript from the following web pages respectively.
http://www.youtube.com/swf_test.html
Link Removed - Invalid URL
(2) After you restarting the browser check for the addons required for playing the video are enabled or not. To view them goto Internet Options, select Programs Tab and click on Manage Addons.
(3) Try by uninstalling any newly installed softwares or some software which you think those might causing problem or try by System Restore.
And, for 64 bit browsers the Addons are not available yet, try 32 bit browsers.
I hope these help you.