Phiran,
I'm running PHP 4 RC2 on a dedicated NT box under IIS right now and I don't have a problem unless I try to run PHP as an ISAPI module.
I see they have better support for ISAPI in the full release so I'm hoping I can set it up properly under NT and have a forum that really flies
There are people who are running it under PHP 3.x right now with no problems. Some of them had to change the file extensions to PHP3 but it runs fine for them. So, in answer to your question, yes, you can change the file extensions.