I need some help securing my home wireless network.

I have a Thompson Speedtouch 4 port DSL modem. One of my desktops is wired into that modem. That computer is not wireless and is not a problem. I have another desktop wired into one of the ethernet ports in my D-Link DIR-655 wireless router. I also have a laptop running wireless from the D-Link.

Here is my question:

If I secure my network using WPA will it affect the desktop that is wired to the D-Link? Or does the D-Link simply operate as a pass through device for that computer? That computer does not have a wireless card.

If securing the wireless network will affect the desktop that is wired to the D-Link I will need to change my configuration. But I really do not want to do this unless necessary.

Thanks for your help.