I am not with Laminar Research; just a flight simmer.
On the basis the card is already installed download the demo version from Laminar Research at no cost other than your time and try it. If it works and you are satisfied purchase the product from Laminar and when you start XP again enter the product key provided at the time of purchase. This will then allow you to access the full version you have previously unknowingly downloaded ie demo version becomes the official version once purchased with no need to download anything else.
Don't waste you money on steam. It is not the official version of XP. It is a different product. Steam will not mix with X-Plane from Laminar Research and vice-versa.