Here we go "mcrypt."
My first attempt at installing Magento and I'm saddled with a mcrypt message that does allow me to install until I correct this.
From what I've read mcrypt is deprecated 2007 but it doesn't look like Magento has kept up with this.
I'm running php7.2.5 on Xampp until I get to grips with everything.
Can anyone throw any light on this subject? it's driving me mad.
Latest stable version of Magento won't work with PHP 7.2.x yet.
There is a project on going to provide support to that version and also the Mcrypt libary was dropped (Magento will use libsodium).
You can read more about the project here: https://community.magento.com/t5/Magento-DevBlog/From-PHP-7-0-to-PHP-7-2-in-Magento-2-3/ba-p/91371
Also, you will need to downgrade your php version to 7.1.x in order to get a Magento stable instance working.