Dê uma olhada nesta postagem :
The solution to this problem was to reboot Windows (stopping and starting the Apache 2.2 Windows service also didn't make a difference), even though the path wasn't altered. After rebooting, the ImageMagick-specific lines below didn't cause errors, and the script was compiled and executed successfully and consistently.
One more thing: Make sure you manually download all Windows updates, including right up to the current Service Packs. To make sure you've got all the updates downloaded, keep going back into the Windows Updates system until there are none left to download -- in Windows XP the "Express" mode won't get everything, so you'll need to choose "Custom" and turn everything on (although you won't need the Windows Live stuff).