ok couple of quick simple questions first:
You say the PHP files merely display the page as PHP code text.
Are you for sure open the files through the apache web server? This is kind of an obvious question except i have dragged and drop local filed into my web browser before and been confused for a minute why the php wasn't being interpreted. In other words, the php files, are you accessing them like
http://localhost/myfile.php or
http://127.0.0.1/myfile.php and not file://c:\myfile.php
Second question would be that it sure sounds like your apache httpd.conf file is not saying to interpret php files with the php interpreter. One thing to watch out for is having multiple copies of httpd.conf in different locations and editing a useless one and wondering why the changes don't take. you might want to first confirm that's not happening.
Then of course make sure you restart apache after you make a change.
Lastly, some recent changes made files that used to be marked as php with <? tags now can require <?php tags. see if that's your problem.