PHP provides a number of different versions for install in Windows. There are versions which are tagged with VC6 and other with VC9. There are also thread-safe and non-thread-safe versions of the installer available for download. After some digging and trial & error fiddlery, my conclusion is that VC9 and non-thread-safe are for the Microsoft IIS webserver while the VC6 and thread-safe installers are for Apache and other non-IIS webservers.
Hope this helps :)