Php – How to install a new PHP extension on IIS 7.5

I have installed PHP 5.2.13 through Web Platform Installer on IIS 7.5/windows 2008 R2
Now I'm trying to install additional PHP extensions, such as mcrypt, so I downloaded the compiled .zip file from the php windows binaries site.
I put the relevant dlls in the extensions folder, and added the lines in the php.ini:

[PHP_MCRYPT]
extension=php_mcrypt.dll

After restarting IIS, no application was working.
This was with VC6 compiled/non thread-safe compiled version of PHP 5.2.13 .zip package.
I also tried with the thread-safe version, which prevented IIS to restart and forced me to reboot the server.

What am I doing wrong here?