well if you go to Tools>Internet Options>Advanced>Reset and reset your browser back to how it was when you first installed it. You may need to set up your home page again.
No, what version of IE are you using? It could be that your older browser might not work with youtube. I doubt it though. Unless your are using prior to IE 4 haha.
sometimes the antivirus thinks it is a malicious script and blocks it
that can also be the case
and whether you use IE or Firefox unless the flash player and the shock wave player needed aren't there your problem may not be solved
hope this helps