Hmmmm, this is something I am not totally sure about - however - it seems the only way your neighbor could have gotten your connection information is if you are wireless. Is that correct?
If so, I am not sure how to protect yourself. I know of people who park in front of people's houses that have wireless internet, so they can have free internet connection, (yes, such upstanding citizens) So I know it is easy and very possible to access other's internet connections. However, I am very sure that in order to access your personal data on your computer they would need a password, etc., so I do not think your personal docs are at risk.
If this still does not answer your question, call a local computer repair company and ask them. Those computer geeks (said lovingly) know absolutely everything about this stuff, and love to share.