Moved thread to Install forum. While we're here... have you tried reading through the FAQ stickied at the top of this forum? I've posted a "how-to" for installing PHP when using IIS6. You might try reading over that in case there's a step you've missed?
If nothing else, try removing all references of PHP you've added to IIS and follow the how-to guide.
Also, when you say you've "checked permissions" ... what does this mean? Did you ensure that the IUSR account has permissions for the .php file in your doc root? Have you also ensured that the IUSR account has at least Read/Execute permissions for the folder you installed PHP in?
Speaking of PHP folders, what's the path to the folder you installed PHP in? Does it have spaces?