cannot access wireless network using ethernet adapter

Hi can anyone help?
I am trying to get onto our home wireless network using a G4 a friend has just given me. It doesn't have an airport facility so my friend has also given me a 802.11g wireless ethernet adapter. I have followed the instructions exactly (accessing the adapters internal configuration and setting it to the wireless networks SSID and channel, then going back into the network settings and re-setting it to "obtain IP address using DHCP') When i restart the computer the network status reads as "built in ethernet is active and has a self-assigned ip address. you may not be able to connect to the internet" (the marker next to it is yellow)
needless to say I cannot connect to the internet. I have tried setting a manual ip address, one within the range of our wireless address (192.168.2.5) and then the network status reads "built in ethernet is currently active. you are connected to the internet via built in ethernet" (the marker next to it is green)
But even then when I open safari it says "failed to open page because you are not connected to the internet"
Any help would be greatly appreciated, I'm at the end of my tether!!