IIS password protection of files and folders has always been dificult.  IISPassword brings the ease and power of Apache’s htaccess to Microsoft IIS. No longer is there a need for system user accounts and complex access permissions for maintaining a secure, password protected web site.

IISPassword is a similair Isapi Filter as JKProtect posted a few days ago.
This one has a nice IIS Admin MMC add-in to adminstrator users. 

IISPassword uses Basic HTTP Authentication for password protecting web sites on IIS, just like htaccess works on Apache. That makes your password protected Apache web site compatible with IIS, and vice versa.

A powerful and intuitive interface makes it possible to password protect a web site in just moments. More advanced settings provide options such as user group management and protection of certain file types.

All for free
IISPassword is free for private, educational and commercial use.
Troxo IISPassword Website

Download
http://www.troxo.com/downloads/direct/?direct=1