nowireless

I have a really annoying problem. I have one computer in the house which will only connect through the wire and not wirelessly. I have a Belkin Wireless G Desktop card which can see the network and reports that it has maximum strength but cannot connect. The computer has no problems connecting when it is wired in. Other computers in the house have no problem with the wireless connection. I have checked the internet settigns and the Encryption and cannot see anythign that is different on this computer that the other which connect. The fact that the computer can connect wired but not wirelessly suggest that there is something wrong with the wireless card - is this a correct assumption and if so how do I test it? Bizarrely the computer used to work intermittently wirelessly but has now given up entirely. Any suggestion greatly appreciated. Many thanks