Xbox 360 Wireless Adapter?

I am just now getting an Xbox 360 Wireless adapter, and I am very confused with the whole thing. I have an e wire modem that allows us to connect the downstairs computer to the internet. Is this the same thing as a wireless router? Also, I have the E-Wire Modem for Windows XP, but I don't know how to access all of the Pwords and codes you need to set up the wireless adapter for the 360. I know I sound like a caveman, but please, someone help me! I just can't live without playing Halo 3 online! Thank You!