Wordpress won't load after updating PHP to 5.4, gives FastCGI Error (Windows)

I'm having a lot of problem here trying to start the development of a new website. We have an external host running on Windows (our client already have a website on the root - we end up not having control over the server environment). I installed Wordpress through FTP and installed the db, and everything was fine and working, but the website was REALLY slow, and I was trying to install a template but I was getting some errors suggesting a lack of capacity from the server. So I used php.ini to "upgrade" some settings, like memory_limit, execution_time etc. After that, I was finally able to install the template.

However, I couldn't install some plugins, and the development was really slow... at some point, Wordpress warned me the server was running PHP 5.2, and that I should update it (of course). Unfortunately, as I said, we're using an external host with its on system and dashboard, and I had to use that dashboard, so the host would automatically take care of the update. Again, I have no control over the environment.

When I tried to update it (I tried several times, to versions 5.4, 5.5 and 5.6), the Wordpress stoped loading at all, every page and access, and it displays IIS errors... I'll paste some text from these errors below:

HTTP Error 500.0 - Internal Server Error

c:\php56\php-cgi.exe - The FastCGI process exited unexpectedly

Detailed Error Information:
Module     FastCgiModule
Notification       ExecuteRequestHandler
Handler    PHP-FastCGI
Error Code     0xc0000005
Requested URL      http://www.agiltecnet.com.br:80/novositesmall/wp-admin/index.php
Physical Path      E:\home\agiltecnet\agiltecnet.com.br\web\novositesmall\wp-admin\index.php
Logon Method       Anonymous
Logon User     Anonymous

after getting support from the host technicians, it started to display this error:

HTTP Error 500.0 - Internal Server Error

<handler> scriptProcessor could not be found in <fastCGI> application configuration

Detailed Error Information:
Module     FastCgiModule
Notification       ExecuteRequestHandler
Handler    PHP-FastCGI
Error Code     0x80070585
Requested URL      http://www.agiltecnet.com.br:80/novositesmall/wp-admin/index.php
Physical Path      E:\home\agiltecnet\agiltecnet.com.br\web\novositesmall\wp-admin\index.php
Logon Method       Anonymous
Logon User     Anonymous

Can someone please shed a light on this?