Basic Authentication with IIS

With apache, you can easily match up a L/P to an htpasswd file. With IIS, the user has to be created as the windows level. I'm moving a site from an apache server to an IIS server, and need to figure out an easy way to do basic authentication. I guess putting a chunk of code at the top of every script to check headers would be appropriate, but that would be a lot of work for this site, plus I have PHP scripts that I didn't write and are very complex and trying to edit them in order to accomplish this would be hell. What other alternatives do I have for basic authentication on IIS? Is there a way maybe to make users on the O/S and just give them minimal permissions? If there's an article somewhere on this, just give me the link =) Thanks for your help!