Depends on which encryption your neighbour uses.
WEP, WPA or WPA2? WEP is easy to crack, WPA and WPA2 are difficult and it's not really a crack.. more like bruteforce.. could take you months
However, you'll need a Wifi card that allows packet injection and i would recommend a Linux installation or livecd.
Take a look at backtrack, it's a linux live distribution just made for stuff like this.