I had this same problem yesterday and found an error in my changes to php.ini file. See page 14 in in Chapter 2.
I started with php.ini-dist.
Copy this as php.ini to c:\windows.
Make sure you have not made a mistake in changes to php.ini as per page 16. I had an error in extension_dir = "c:/php/ext (note forward slashes)
See also this URL http://www.cs.wcupa.edu/~rkline/apache-php-win.html
The author has some additional changes to php.ini that I followed.
Hope this helps