I'm no expert but it seems like maybe your httpd.conf file has gotten corrupted or changed. If you are running similar versions to mine, check your C:\Program Files\Apache Software Foundation\Apache2.2\conf\httpd.conf file for the lines like this:
and also maybe this line:
#BEGIN PHP INSTALLER EDITS - REMOVE ONLY ON UNINSTALL
PHPIniDir "C:\\Program Files\\PHP\\"
LoadModule php5_module "C:\\Program Files\\PHP\\php5apache2_2.dll"
there should be a file called httpd.conf.bak that is a backup of that file, so you might try copying that file over.
DirectoryIndex index.php index.html index.htm
Also possible that there is a problem with your PHP installation. Easiest thing to do would be to uninstall and re-install it. An easy way to test that PHP is being processed is to put the info.php file into your htdocs folder. (sometimes called phpinfo.php) and try to access it.
Hope this helps...