would have to agree with waddy, I've heard nothing but complaints/problems when trying to install PHP under IIS. Give PHPtriad a go and you will have better luck.
I was looking for a particular site that had really good instructions on how to do this but I could not find it. This one looks pretty close though. I have not done a lot with PHP in windows using IIS so I dont know if there are the problems like the others said but hopefully this helps. Good Luck!