This is more a question for the Installation problems forum, but...
Have you thought about installing something like EasyPHP - this saves a hell of a lot of grief with getting the setup right.
However as Matthew says - a 'vanilla' install of Apache won't work without some modification - the main thing is to search for DirectoryIndex in httpd.conf, it should at least look like:
DirectoryIndex index.php index.html
...you will certainly have to add the index.php one. Also make sure the folder from which web pages are served is readable by the web server. In Windows the file is probably Read/Write by everyone as default so this is probably not the problem.