Mac users need password to logon webpage

I have a NT server running IIS 4, default website is for the library in a school. We have Mac clients used to just type in the IP address/website page and go right in without any problem. Lately, when we type the IP address/page in the browser, a logon window popup asking the user name and password to logon to NT. We also have PCs, and they have no problem. As far as I know, nothing was changed. I checked the Properties of the default website, Anonymous user authentication was checked. Please help. Thanks.