Which type of encryption are you using on the W-LAN? WPA? Fine and dandy - even if you happen live in the Tora Bora mountains. WEP? Any wardriver with a modicum of knowledge and the readily available tools has a high chance of breaking that and once he has access to the LAN, he has access to shared files. But he's not after your files; he's after free broadband internet. He hasn't got the inclination to browse through your files unless he thinks that you might be daft enough to store a plain text list of passwords or bank details in a shared folder.