EDIT: Update - scroll down
EDIT 2: Update - problem solved
Hi this comes a little late, I've wrote a spawner for php-cgi.exe on windows, not perfect but it might be what you needed. Check it at here.
Okay, I managed to find the cause of the problem. It wasn't my code at all. It's PHP, it cannot spawn additional php-cgi's under Windows when running as FastCGI mode, under Linux it works perfectly, I simply pointed my server to my linux box IP and it had no problems with concurrent FCGI requests. Sucks, but I guess that's the way it is...
I did look deeper into the PHP source code after that and found that the section of code which responds to PHP_FCGI_CHILDREN has been encapsulated by #ifndef WIN32 So the developers must be aware of the issue
re: spawn-php python script...
Thanks @nosam that really helped.
For those wanting to get it working quickly you'll need the following (if 64bit system)
ActivePython-2.7.2.5-win64-x64.msi
pywin32-217.win-amd64-py2.7.exe
ActivePython does not have older versions of these on their www so you will need to do a bit of googling around to find a working mirror (there are plenty out there)
Once you have downloaded the src from bitbucket you may need to edit spawn-php.py (to fix up the tab spacing), as bit-bucket seemed to mess up the tab's in the file preventing it from running.
All-in-all that saved my day for a busy little windows website using nginx + fast-cgi.
Thanks mate!