[thelist] IIS "HTTP 401.3 - Access denied by ACL on resource" Error

I am trying to set up my IIS web server to let anonymous users in, but it
keeps asking me for a username and password.  If I choose not to enter this
information (by clicking the cancel button, obviously), it returns this
error:

HTTP 401.3 - Access denied by ACL on resource
Internet Information Services

I have had this problem before, but only when I had tried to browse the site
using Mozilla.  Now even Internet Explorer won't let me in without a
password.

Has anyone else had this problem, and if so, could you point me in the right
direction to fix this?

Thanks in advance,
David Bindel