As per this
https://www.whatsapp.com/faq/en/wp/21478428 As of July 1st, 2016, the requirements to begin using WhatsApp on a Windows Phone are the following:
[LIST=1][*]The phone must be running Windows Phone 8 or higher.[*]The phone must have an active Internet connection.[/LIST]
Depending on who you believe it MAY or MAY NOT be possible to update your Lumia 800 to the required 8.1. I have read conflicting reports in my googling. I would also caution you, since your model is an older one and not designed for Win 8.1 or Win 10, even if you CAN get it to install it may run poorly. Do you REALLY want to do this for 1 app? What about going to another messenger platform? Skype, BBM, or Kik?