Got it! Although I've almost forgotten why I was trying to install it in the first place!
You were right, Redcircle and ruturajv. I uninstalled PHP, unzipped the PHP files to the C drive, put a fresh copy of php.ini in the Windows folder, got a fresh copy of httpd.conf, followed the manual installation instructions at php.net, restarted Apache, and there is was!
Thanks for your help.