W2K Pro web logon problem

I have an IIS web server set up on a Win2000 machine, for a web site and FTP site. Things were fine for a long time. You could access the site anonymously or login as a user. But recently whenever you try to access the server by FTP it throws up a login window, and won’t accept any logins, including FTP. I can’t think of anything I did to cause that. The web part works fine, including the ASP pages. I’ve reinstalled IIS, turned the permissions down to nothing both through IIS and the security and sharing sections of computer management, shared everything, included every possible user and group, and nothing changes.

Does anyone know what’s wrong?