Installation issue with PHP 5.3.1

I am about to throw something very expensive through a large window right
now.

Here's the issue:

PHP version: 5.3.1
Server: IIS 6
OS: Windows XP SP2

I had PHP running fine on my development machine for months, but then I had
to write a script that allows users to upload files to my site.  Due to the
ridiculous security settings for IIS, I got frustrated and decided to switch
to Apache 2.2.14.

COULD NOT get PHP 5.3.1 to work with Apache (the normal php.ini lines worked
fine, except for PHPIniDir - this caused Apache to crash on startup.  If it
was left uncommented, it worked fine), so I'm trying to switch back to IIS.

I've completely uninstalled PHP from my machine & removed any old files.
 When re-installing using the msi, I get the following error message: "Error
trying to find the nodes containing scriptmaps :GetDataPaths Failed.  You
will need to manually configure the web server."

I then manually configure the server to use FastCGI, according to the
instructions on the PHP website.

After restarting IIS, I navigate to the test page for my site.  Instead of
displaying the page, my browser downloads the page as a file.

Does anyone have a clue what's going on here?

-Matt