If you saw 60 Minutes last Sunday (about the conficker worm) then you would rightfully be very nervous about on-line banking or purchases. 60 Minutes showed a person with up to date virus and firewalls who got infected. The worm is set to strike April 1, 2009. This article has some good advise:
http://blogs.computerworld.com/the_conficker_worm_on_60_minutes
More info on conficker worm: http://www.eweek.com/c/a/Security/Conficker-The-
