If you have a young friend, who is really tech savvy, you would be surprised. My Grandson, now older and working in Internet Security for a defense contractor, once sat there on his computer and told me every website that I was looking at, and where I had been. He also told me that he could hack my PC and locate the file with all of my passwords, etc. He did all of this on a supposedly secure wireless network.
There is a reason why info is lost to hackers every day, from big places like banks, etc. It is not that difficult for someone to do if they know what they are doing. 99.9999% of us do not have anything to worry about but I can tell you from experience that if someone is using the same wireless network as you, and you think you are secure, because of WEP, or other security encryption, you are not. The odds are very low that you will be targeted, but it can happen. Sort of like the NSA monitoring your phone calls. They can do it but it is very doubtful you will be their target.