executable in another web page

Hi folks
My setup is as follows: O/S= XP, Web service = IIS
I have 2 seperate web pages as follows.
(I will need to put this onto Win2003 server eventually, but I'm a real newbie to PHP and MySql, so just trying to get familiar with the environment onmy PC first.)

Page A - is a macromedia flash(2004) based website which launches a text editing tool (TextEditorName.exe - apparently runs a cgi script) so that the user can edit the web pages displayed. (I.E. This page does not require PHP and has been working well.)

Page B. Recently our client asked for a bulletin board. This has been developed using PHP and MySql.

Now the problem is that as soon as I load PHP onto the server (Immediately after installation - No Ini changes etc) then the editing tool in Page A stops working - Displays a "HTTP 500" error.

I think that I have made all the suggested changes as found on the FAQs. Eg;
- Updated parameters/paths in the PHP.INI.
- Granted permissions in IIS (Scripts & Executables)
- "Admin/Services/server/properties/login/Allow interaction with desktop" is selected.
- Anonymous access is granted.
- I have shared the relvant Folders & Files.

I still cannot get the text editing tool to work.

Does the PHPInstall.exe process change settings within IIS or is the problem perhaps that PHP is now somehow overiding the default windows CGI script intepreter?

It may sound like I know what's going on, but actually its taken me about 5 days to get this far.

Any other suggestions would be more than welcome.
Thanks.