Rich,
The machine I had the problem on is running WinXP with the network firewall (advanced tab) disabled, but it is running McAfee firewall (latest version). Also, the secutiry settings for IE were set to the default medium level. I'm not sure if this has anything to do with the problem but when the error occurred, the cursor in the editbox for entering post text was flickering very quickly, as if it was caught in some fast running loop. I'm using that machine now to post this, so if this fails, there must be a problem with the firewall or security settings.
Paul.