When you open outlook express, click on tools on the top menu bar. When the menu drops down, way on the bottom is an option for Accounts. Click on that and then click on the "mail" tab.

Highlight and "remove" any accounts associated with yahoo. Then create a new account with the Netzero settings. You may need to call them to get the settings. Once you get the Netzero account set up, make sure it is the default setting.

This should work. If you're still having problems, let us know, but once you delete the Yahoo accounts it will at least stop looking for that account and giving you an error.