After much hair pulling trying to guide a family member over the phone, I found this link...
...which I found easier to follow than the many forum posts with similar advice.
It seems a full uninstall and clean install is the only solution to the v8 upgrade blues.
One difference from the steps provided above, was that I had to carry out all the steps in Safe mode. In normal mode truevector kept getting in the way.
To get to Start mode, rather than use the F8 on boot method, I used Start > Run > msconfig > boot.ini tab > safeboot checkbox.
I tried the uninstall utility from Zonealarm but it kept closing with an error message. In the end I didn't need it.
Thanks to the forum member who provided the above link (now lost in a sea of browser tabs). I hope this saves someone from a bit of timewasting and hairpulling.
Operating System:Windows XP Pro
Product Name:ZoneAlarm (Free)