Problem with IIS and PHP files

I'm having a problem with phpBB Forum software, but I think the problem is with IIS config for php, rather that phpBB itself.

I have already a copy of phpBB running, but wanted to split the Forums into two separate 'boards'.

So I created a folders

D:\WebSites\dualforum\forum1 and
D:\WebSites\dualforum\forum2

I configured IIS for a new website DualTest, pointing to D:\WebSites\dualforum\ as the Home Directory.

I then installed all the phpBB files as per the first BB and tried to set them up, but I couldn't get the browser to find index.php ("The page cannot be found").

So I created some test.htm files with just the name of the folder and put a test.htm and a test.php (containing just <? phpinfo(); ?>) in various folders. Then tried to access them.

If I browse for test.htm or test.php in the root folder (e.g. my.domain.com/test.htm), they both work fine. However if I browse for them in my.domain.com/forum1 or my.domain.com/forum2, it finds the .htm OK, put says it can't find the .php!

Now I'm confused. PHP must be working or it wouldn't work with the root test.php file. IIS must be finding the forum1/2 folders OK or it wouldn't find the test.htm files. So what might I be doing wrong that is preventing it from working with PHP in the subfolders??

Some sort of php.ini setting that I am missing??
UPDATE: I'm beginning to suspect that the culprit is the doc_root setting in PHP.INI! But I'm not sure exactly how?

I run several php pages on my single web server... I don't have doc_root set, and have the cgi.redirect set to 0...

Are you allowing scripts to run in your IIS settings for the new site? (on the home directory tab for you website's properties, make sure that scripts are allowed to execute)...

Another thing is to make sure your php extensions are valid in your new site (i.e. did you define them at the root level, or on the site level?)