PHP with fastcgi (IIS 6)

This isn't MySQL related but since I also run PHP on some Windows 2003 servers (IIS 6) I thought I would make a post as I switched from isapi to fastcgi.

The process went fairly smooth following the good instructions at iis.net on my dev server. Here is the link to their article:
http://learn.iis.net/page.aspx/247/using-fastcgi-to-host-php-applications-on-iis-60/

When I deployed to one of my production servers I was having issues with a specific application. I was getting this error:

FastCGI Error

The FastCGI Handler was unable to process the request.

Error Details:

* Error Number: 5 (0x80070005).
* Error Description: Access denied.

HTTP Error 500 - Server Error.
Internet Information Services (IIS)

All of my applications work on their own application pool. Since all the other applications were working properly I knew it was not permission issues to php-cgi.exe like others have posted about. My issue was application pool specific. The difference this application pool had vs the others was in the properties > performance for the pool. Enable CPU monitoring was checked. Once I unchecked this everything started working properly.

Issue solved. I just want to read up now on why this doesnt work and what other issues are related in iis 6