My desktop is over 4 years old too and so is not wifi enabled.
I prefer to keep mine like that and connect the desktop to the router by ethernet (I had to buy an ethernet card as my pc didn't have the right connection. �4 and easy to fit), and the laptop connects wirelessly.
They both run fine, and are not slowed down at all when both are used at the same time. This is precisely what a router is designed to do, after all.
If I wanted to, I could buy a wireless adapter for my desktop and use it wirelessly, but as it is sat next to the router it is pointless.