They're both considered anout as secure. Radius is just a more random password generation.
You will also want to allow ONLY your mac address. Bit of a pain if a buddy turns up and wants to use your router, but it's only a few more clicks to add him.
If you use your network card's software instead of the XP wireless wizard, you sometimes get the choice to ONLY detect on a certain channel. Netgear is one such product.
It's not nice, but you could possibly read up on connecting to that person's wireless router and reset it for them.
If they haven't bothered to secure it, chances are it's still broadcasting the default SSID and from that you can see right away what type of router it is.
Most manufacturers use the same admin login and password on all models.
(As you can see
here)
You'd actually be doing him a favour if you secured it and 'turned down the volume' for him.