What version of Apache? There should be a sub-folder in the apache root directory called modules (eg. c:\apache\modules). Copy php4apache.dll into that folder and change the LoadModule directive for PHP in httpd.conf to the following:
LoadModule php4_module modules/php4apache.dll
Try to restart Apache, it should have worked itself out. Typically I try to avoid installing servers/languages into C:\Program Files. The longer paths (especially those with spaces) tend to make life more difficult than necessary.
Also, for the time being, make sure you're not loading any additional extensions (ie. extension=php_*.dll) in php.ini. Some of them can occasionally cause mod_php to bork on Apache. Once you've got'er up and running you can play with loading the additional extensions.
-geoff