If you are setting up the enterprise portal for claims users you might want to turn off the authentication selection when the user first accesses the site. Unless you are going to train the users what to select. If you require internal users to access the portal and use their regular domain accounts then you might want to setup a separate site just for the windows users. Then you can turn off windows authentication on the claims site so the external users don’t get prompted for the authentication selection. Here is a quick walk through changing that option.