I'm not familiar with hosting php websites in windows but I know for IIS hosted websites I get this error for a myriad of reasons. Some of which you've mentioned above. When hosting in IIS I know there is a setting that needs to be set to True called "Enable 32 bit applications"
Also you want to make sure if you are transferring this file from another computer that the file is unblocked as this will cause a lot of headaches as well and strange errors.
Sometimes what I do as well is to write a simple test gui that calls this com dll to eliminate anything related to running it as a website. If you are able to access it with the gui application it could be something security related to running it as a website if the user the website is running under is different than the user launching the gui application.
Also try the article mentioned here:
It seemed to have helped you judging by the comment below.