ImageMagick and FastCGI
I have just been trying to get a Media Wiki install to use the ImageMagick conversion library on a Windows/IIS stack for the purpose of SVG thumbnailing.
In true open-source form, the documentation is weak. When I attempted to run the conversion I would receive an error from FastCGI "Unable to Fork" when using passthru().
I knocked up a PHP script to see if this was a standalone issue or all shellexec statements. Nothing worked.
Whilst ImageMagick is a standalone executable it needs to run in a command shell environment, so for this to work, the calling process needs to be able to execute "cmd.exe". Adding the appropriate IUSR_* account to be able to read/execute cmd.exe fixes the problem.
NB: you can do a lot of damage with cmd.exe; by allowing your web-server to execute this opens a large attack vector to your server.
If you're looking for a method of converting images, then probably using PHP extensions will be safer.