The usb adapters usually allow you to connect the device they are plugged into wirelessly to a wireless router. so in plugging the USB adapter into your pc, you are in fact allowing THAT pc to connect to a wireless router, not making that PC an access point.
if you want to use the wifi on your ipod to connect to the net, then you will need a wireless router, you can buy these that come with the ADSL modem in, or you can buy just a router and plug your modem directly intothe new wireless router. This is assuming that you are using ADSL broadband and not cable.