I found the problem. I use Zonealarm as my firewall. It has a site in the Privacy/Site List labelled Loopback which, as I understand it, is a program 'looping back' to 127.0.0.1 - i.e my computer. I had the settings for the Loopback set for enabling Mobile Code, Allowing cookie Control Session and Persistent, but not allowing for 3rd party. I changed that last setting to allowing 3rd party for Loopback and it now works just fine.