Well, first, you might want to consider not using one of those unsecured connections, somebody could be reading your files right now. Also, it's technically illegal to use someone else's broadband without permission and you could get into trouble for it.
The most likely reason you cannot see your own network is that since you got your wireless broadband, more people in your neighbourhood have done the same (including the potential scammers/spammers with unsecured 'honeypot' networks) and they're operating on the same channel as you.
Try changing your wireless network to a different channel, say 3 numbers away from where it was.