You can buy some b.s. software like net nanny, or the million variations.
You can also take the cheap way out.
Download a firewall program, there are some available for free.
Under the section for blocked domains, add
http://www.youtube.com, and the rest. Hit ok.
Usually these firewall programs will allow you to password protect as well.
There is a chance that you can do this in your router's configuration as well, if you have a router.
Remember that depending on who is using your computer, they may be smart enough to reset a router or install a key logger to see what your passwords are.
