Set the NTFS permissions on the directory in question, and then set the directory security mode in IIS to clear text password:
1. Open IIS Console
2. Expand the site in question and right click on the desired directory, then select properties
3. Select the Directory Security tab
4. Select "Anonymous access and authentication control"
5. Select "Basic Authentication", while clearing "Anonymous Access" and "Integrated Windows authentication"
6. (Optional) Edit the "Basic Authentication" and enter the NT Domain name (or workgroup name) either directly or browsing to it
That should cover it.