Many thanks Gandalf1369 and Scout321x for your suggestions and for taking the time to help. I've followed your advice and checked Zone Alarm; Avast was listed in the program access area - I cleared this and then installed Avast 5. Restarted and same problem as before, so then restarted in safe mode, removed Zone Alarm and tried again, success, windows booted and the system tray loaded.
Only problem now is that I have a Climate Change Experiment running on my computer (BOINC BBC Climate Change Experiment) which runs at start up. This now takes a long time to initiate and appear in system tray (whereas didn't before), though when it finishes I find that the progress of the experiment my machine is running resets to zero (had been on 40% after several thousand hours of running) - fortunately I have a backup so this isn't too bad, but on copying over the backup data and restarting my pc same thing happens, experiment is reset and progress is zero. Avast appears to try and start during Climate Change Experiment initialisation which may be the problem ?
Also initially I get a cross, then warning triangle on the orange Avast 5 icon in the system tray which says mail and web shields are disabled, this changes after a bit to fully protected, but is this normal - didn't happen with v4.8 ?
Though I liked Zone Alarm I could have lived without it and used windows defender if it meant Avast 5 worked, but Avast now appears to be interfering with the climate change experiment which I've been running for some years and don't want to give up on. I can't be having it reset every time I start my machine - never happened before with Avast 4.8. As it no longer seems possible to download v4.8 from any sources I trust I'm probably going to have to use different anti virus software from now which is a shame as I loved Avast, but I can't be doing with sequential failures of operating system and applications.