As mentioned in the previous entry, I updated Zone Alarm after high DPCs caused by version 7.5s outdated True Vector service (vsmon.exe) caused my system to become unusable. The problem disappeared and everything was fine until a new problem occurred: Firefox would hang after ~30 minutes usage, refusing to load new pages. Firefox would also refuse to quit or start again, requiring a restart of the computer to fix this
I did a little Googling and the evidence seemed to mount up against the new version of True Vector. Suggested solution: use a different firewall. Uninstalled Zone Alarm. Installed Comodo Firewall. Using it in Custom Policy Mode pretty much replicates Zone Alarm's pop-up allow/deny dialogue (with the minor niggle that the 'allow permamnently' tick-box's state is persistent (from a previous answer) instead of always defaulting to 'do not remember this answer' - and so allow or deny on a case by case basis unless a deliberate exception is chosen.
Firefox no longer hangs. Seems to confirm that True Vector was indeed the culprit. Silly Check Point.
The Comodo firewall seeemed to have issues with Avast! antivirus (not exactly confirmed - it caused running firewall after initial reboot to fail) so I uninstalled Avast! (which I liked) and used Comodo's antivirus - along with the entire Internet Security Suite (using the built-in Defence+ instead of Windows Defender). I do like all the information that Comodo gives me (details of every connection etc.) although some aspects of it are slightly non-intuitive (especially compared to ZA's user-friendly interface).
One day and one night running successfully. Early days.