Are you getting a login screen in the browser or as a popup dialog off the browser?
This sounds like a security setting in IIS. By default, new web sites and virtual directories in IIS are set to "Integrated Windows authentication". This prevents a user from even BROWSING a site/directory until they have provided a valid windows user login/password. This has nothing to do with the site login you programmed. You need to allow anonymous access to the site so that anyone can browse the pages.
Open IIS Manager, choose properties from the context menu of the virtual directory in question, select the "Directory Security" tab, under "Anonymous access and authentication control" click the "Edit" button, uncheck "Integrated Windows authentication" and check "Anonymous access".