Lets make this simple!
First, go to the IIS administration panel and disable the thing (better yet, remove IIS off your system). This, to avoid having two HTTP servers sharing the same port.
Second, download Apache 2.x and install it. Make the following changes in the Apache Group/Apache2/conf/httpd.conf file:
AddType application/x-httpd-php .php
PHPIniDir "C:/Php"
Now to make sure Apache is working... look near your system clock in the status bar. There should be a apache icon (pink feather). If the arrow is red, you don't have Apache started. Click on the icon and start it. Then, go to your browser and type in:
http://localhost
You should see the Apache page come up. If not, you have some problems and need to troubleshoot Apache. Do not continue until you solve this problem.
Third download the current 4.x version of PHP and install it in the C:\Php directory. Copy the file php.ini-recommended and name it php.ini
You shouldn't have to monkey with the PHP.INI file at this point (read the manual and monkey with it later to fine tune it).
Fourth, go to your Apache Group/Apache2/htdocs directory. Create a new folder called Test. Inside the test folder, create a file called PhpInfo.php:
<?php
phpinfo();
?>
Now go to your browser and type in:
http://localhost/test/phpinfo.php
If you see something coming up, congrats, you got PHP installed correctly.