That's useful info Mick.
I moved from Norton and ZoneAlarm onto BitDefender last year and so far, touch wood, no ill effects from the latest XP (SP3 and the aforementioned two) Updates.

Googling ZoneAlarm, I see, now brings up the following advice: "Check Point has promised to provide a fix later today to restore internet connectivity to ZoneAlarm customers who have suffered a web blackout after ..." and quotes VNUNet.com as a source.