I doubt the wireless network was breached. More likely is the presence of some sort of malware on the machines. Have you already run scans with SuperAntiSpyware and MalwareBytes? Also, full scans using antivirus (possibly two different AV programs)?
Suggestions in general (and probably stuff you may already know): keep your antivirus, OS, and browsers all updated, run anti-malware software somewhat regularly, and don't visit sketchy sites. If you really want to lock down the browser, you can run NoScript in Firefox, but I would not recommend this to a novice user. Also, Symantec is not the best AV out there. Some free alternatives are Microsoft Security Essentials, Avira, and Avast. If you have not already done so, change the passwords for other email accounts.