This needs a little clarification. MSN operates as a web site and a dial up ISP. They allow lots of people to access the web site, and the affiliated Live and Bing sites, through other ISPs. Some Live services, including Hotmail and personalized MSN/MSNBC pages, require a user ID and password.
Before I switched to a local cable company's broadband service I used MSN's dial up ISP for years without installing the MSN Internet Service software. It was little more than a custom web browser that used Internet Explorer's HTML engine to fill the application pane(s). A few simple settings in the operating system's dial up networking and all the MSN services could be accessed with any web browser the subscriber preferred.