While I appreciate the particular difficulties this incident has caused for those responsible for networks of multiple computers and for those who rely on their computer(s) for business purposes, as a domestic user I am grateful that Avast have resolved the issue in relatively quick time.
I found my way to this thread via the Microsoft Community Forum which quickly alerted me to the fact an Avast update was responsible. I then waited some 7½ half hours until 17:30 GMT before starting up my Windows 7 desktop. I then found the virus definition updated immediately and the system has been running trouble free for over an hour since. Previously, after approximately 5 minutes, the system would crash. I did not exactly panic but was nevertheless rather concerned and tried several things to try to resolve the issue but none, of course, worked. Using my Puppy Linux / XP with internet disabled dual boot system, I eventually posted a query on the Microsoft Community Forum and received a reply very quickly. This proved very reassuring as I realised that my Windows 7 system had no lasting problem. The link to this thread also made me feel more confident as it was very clear it was not 'just me' suffering.
All seems well now so thanks to those who sorted it. Hopefully Avast will be more careful with future updates so that the frustrations and major inconveniences reflected in many of the posts here, are not repeated any time soon. My relief makes me feel quite forgiving but many posters here will rightly be peeved with Avast for a fair while.