Err, that's interesting and all, but nobody sets up their routers with MAC address filtering, and nobody needs to fiddle with the wireless settings on a stock Windows install (and certainly not on the Aspire One: I know because I have one).
If he set up the wireless router himself, or his flatmate did, who knows how it's configured? Some people do use MAC address filtering. It doesn't add much real security because MAC addresses are easily faked, but not everyone knows that, and even the ones who do know may figure it adds a little extra to discourage casual hackers. Similarly it might have unusual settings for other security features. Hopefully it uses WPA2 and needs PSK password and, eg, you need to be consistent about whether its in hex or as a pass phrase. There are lots of things that can go wrong. It doesn't always "just work".
It can be worth switching all security off on the router, and then trying to get Windows to make a connection from scratch. If that works, you know the hardware is OK and you can try re-enabling security. Maybe leave any PCs or other devices offline while you try this, just in case, and of course don't leave the security off.
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