Try downloading firefox.. goto youtube.com, it will ask you to download flash player, firefox plugin should open, press next and let it install, now what?
This problem could be of your bowser Internet security level on high or you didn't restarted your PC after installation. change you bowser setting to medium or download a newer version of browser you are using. otherwise it could be problem because of virus in your system.
Make sure you have IE 8. Clear the cache, cookies, temp internet files. Then try IE 8 again. Although as stated it might be better to stick with Firefox or Chrome.