Perhaps you should try clearing Internet Explorer's cookies and then see if you're able to access Yahoo! or other secure websites.
If you would like to clear your browser's cookies and you're using Internet Explorer 8, you can do so by going to Tools and then to Internet Options.
From there, you should click on the "Delete..." button under the "Browsing history" section of the "General" tab, where you should be able to delete things, such as your temporary Internet files, cookies, history, as well as other things.
If that doesn't work, perhaps you should try repairing your connection by going to the Control Panel and then to Network and Internet Connections.
From there, you should right click on your connection and choose "Repair".
If you are still unable to log into Yahoo! or other secure websites, perhaps you should try resetting your router by unplugging it for about fifteen seconds or so and then plugging it back in.
Good luck and I hope I helped you!