Try the code in Answer: How can I login to get data from a webpage? as a starting point...
I took a look at the source code for the page that you gave... in your case, the "GET" method should be replaced with the "POST" method... and the login URL is http://login.yahoo.com/config/login
What you need to do is to construct the values for the variables within the login form (ie: login, password) and POST it into the form URL... Thereafter, if the login is successful, a simple web page fetch should work for you..