Have you inserted the registry entry to tell it what program to process php files with?
If yes, ignore rest of this post.
If not,
Run Regedit.
Navigate to:
HKEY_LOCAL_MACHINE/System/CurrentControlSet/Services/W3Svc/Parameters/ScriptMap
On the edit menu select: New->String Value.
Type in the extension you wish to use for your php scripts. ex: .php
Double click on the new string value and enter the path to
php.exe in the value data field, followed by %s %s.
eg: c:\php\php.exe %s %s
hth