When leaving my computer I engage the internet lock, twice now in two days opon returning to the computer zonealarm is turned off, it is not in the tray. Nobody has physical access to the computer but myself. Virus definitions are up to date. I've ran zonealarm, avg, webroot, spybot, and adaware. What could shut down zonealarm?

Operating System:Windows XP Home Edition
Product Name:ZoneAlarm Internet Security Suite
Software Version:6.5