There was a thread a while back asking about a non-threadsafe build of Zend Optimizer:
http://forums.zend.com/viewtopic.php?f=57&t=270&start=10
There was even a test build posted by one of the Zend team:
http://forums.zend.com/viewtopic.php?f=57&t=270&start=10#p6058
Does anyone know if there's an official non-threadsafe release for Windows yet? The build linked to above just kills the web server unfortunately.