Login's Not Working

Me, my mother and my dad are Administrators on the home computer. We have no problems but when my brothers is logged in and he tries to get on the internet he can't log in to any sites. For example he can go to the facebook main page (www.facebook.com) but after trying to log in it gives him an error page. So far he has had the same experience when trying to log into YouTube, and Yahoo!. What is wrong? There are no blocks in the Tools>Internet Options menus that I found, when trying to find the Hosts file in the drivers folder (C:\windows\system32\drivers\etc) I couldn't find it. This is what I usually use if we need to block a site from use on the computer. Please get back to me as soon as possible. Any ideas are welcome.

Thanks