don't install spamware/adware/crapware, so don't install anything from yahoo, and NEVER install a toolbar... from anyone. that's step one.
step 2, go to control pannel > programs and features (add/remove programs in xp) and delete the toolbar.
step 3, go to your web browsers (every one of them) and remove the add on
step 4, dl ccleaner, run the regestry sweeper, and clean your regestry, then clena out your temp file folders with it.
step 5, search your pc for Brothersoft Toolbar.exe, delete that file
step 6, double check your registry to make certain all references are removed