I finaly managed to remove the Win32.yok spyware / toolbar. I found an item labeled ' Mail Frontier Desktop', a security filter for Outlook Express. I use Mozilla Thundaerbird and ZoneAlarm 2006 so I never use O.E. I uninstalled it and after a restart, it had dissapeared from ZA's spyware scan, even on 'deep inspection. I'm not sure if this would be an option for those using O.E.

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