Thanks for your response. I diagnosed the worm from the Start Up list (top of this page) but having googled around I discovered this is quite a common problem and seems to occur when ZoneAlarm is the firewall being used.
Checking the ZoneAlarm 'More Info' commentary I discovered that my PC was trying to reach a specific IP address. On checking this IP address I discovered that this is my Wireless router!
Given that ZA keeps blocking this connection attempt I am surprised that I am able to get internet access.
All's well that ends well, etc.