[php] Fatal error: Class 'PHPMailer' not found

I suggest you look into getting composer. https://getcomposer.org Composer makes getting third-party libraries a LOT easier and using a single autoloader for all of them. It also standardizes on where all your dependencies are located, along with some automatization capabilities.

Download https://getcomposer.org/composer.phar to C:\Inetpub\wwwroot\php

Delete your C:\Inetpub\wwwroot\php\PHPMailer\ directory.

Use composer.phar to get the phpmailer package using the command line to execute

cd C:\Inetpub\wwwroot\php
php composer.phar require phpmailer/phpmailer

After it is finished it will create a C:\Inetpub\wwwroot\php\vendor directory along with all of the phpmailer files and generate an autoloader.

Next in your main project configuration file you need to include the autoload file.

require_once 'C:\Inetpub\wwwroot\php\vendor\autoload.php';

The vendor\autoload.php will include the information for you to use $mail = new \PHPMailer;

Additional information on the PHPMailer package can be found at https://packagist.org/packages/phpmailer/phpmailer