How dangerous is it to use an unsecured WiFi network?

My friend Steve just got his first computer, a Windows 7 netbook with WiFi support, and wants to use it exclusively on free public WiFi networks such as those at Starbucks and other coffeehouses, but never at home. He was doing great until a mutual friend told him that if he uses an unsecured (password-free) public network, anyone with a little knowledge can access everything on his netbook, and even plant files on it, such as child pornography which, if it’s found, can end him up in jail or worse. Now Steve is terrified and won’t use WiFi at any location that does not have a secured network. Is Steve’s worry justified?