I had lots of issues getting PHP working, but after trial and error, I finally have PHP up. The PHP.Net community had suggested using FastCGI, so that is the route I went with. I have a Windows 2003 Server machine using IIS6, PHP 5.4.0, and the only FastCGI I could find. Can't remember which version.
Anyway, I finally got RoundCube to at least show the login screen. When going through the initial setup, I was able to send and "login" to my hMailServer without issue. This was on the Test Config screen.
My Issue is that at first I was getting "invalid request. no data sent". Now I'm getting "HTTP/1.1 503 Server too busy Connection: close Date: Fri, 06 Apr 2012 18:51:43 GMT Server: Microsoft-IIS/6.0 Content-Type: text/html Server Error, unable to connect to fastcgi server."
A hint may be that on the "Update Config" screen, I was never able to "update" the config files. I had to manually edit them with Notepad++. I verified that the service that runs php-cgi.exe had the proper permissions to all of the folders within RoundCube, but still have the issues. I don't know if there is a step I missed or not, or if there is an issue with the versions of software I'm using. Any help would be appreciated.