I just hooked up my old computer, and want it to have wireless internet. Now, does this mean I must have a wireless card AND router for the old computer, or do I just need a router to install onto the old computer, and I can hook-up to the already existent wifi I have?

I have installed a dsl modem via ethernet on my new computer but I want to install the same dsl modem on my older computer via usb.I have a cd rom that came with the initial installation.I want to move the modem and software to my old computer.