Ok well if anyone is interested I deleted ZA 7 last night (not clean uninstall, just the quick and dirty add/remove option) and then rebooted, then reinstalled ZA.
All seems ok at the moment, it did an immediate virus scan following an update (something else it used to hang on since the upgrade) and found a couple of "virii" well installer apps actually but at least it seems happy. At midnight last night it again did its scheduled daily scan and went through without a hitch and importantly without locking up the CPU !
Still it's on it's final written warning, one more mishap and it's off for good ! (Shhh it is probably off for good anyway but don't tell it ! )