How to Password-Protect an Entire Site

My company wants to create an Internet-accessible Intranet for employees, sales reps and contractors using an ID & Password that will change monthly.

We are a Microsoft shop, so we are running IIS on NT and SQL Server. What is the simplest and/or best way to do this? It seems that there ought to be a simple, secure solution that is probably right in front of me.

Any and all help is greatly appreciated!