Try to start *Windows* in safe mode with networking enabled.

- Win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode

- Win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode

- XP http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx

Still in Windows safe mode, start Thunderbird in safe mode.

- https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

For the limited time I was able to spend in Safe Mode the problem didn't occur so 'probably' is the answer.

The problem is most likely AVG. Don't let it scan the Thunderbird profile folder. Don't let it scan incoming and outgoing messages. Don't let it scan email attachments.

Thanks but doesn't that leave my system open to harm?

No, you ARE still protected from viruses by other protection features. With the real time scanner turned on it will scan any file or attachment should you decide to open or run it, and it will alert you if there is any infection. Having said that, you should not open or run attachments or click on links in emails from questionable sources anyway.

Thanks again. I am always wary of suspicious mail. By 'other protection features' are they from Windows, Thunderbird or other parts of AVG?

'other protection features'

That's AVGs real-time on-access scanner. It just shouldn't scan the Thunderbird profile folder.

OK. i'll have to check how to do that in AVG and find the TB profile folder.

I have had the AVG real-time scanner excluding the Thunderbird profile folder for most of the morning. Unfortunately I am still seeing the same problem. TB is freezing on a regular basis. Does anyone have other ideas to fix this annoying problem?

If you're unsure how to apply the suggested changes in AVG please contact AVG support.

I contacted them yesterday and they responded very promptly. I applied the exclusion before testing this morning. As I say, the problem is still there even with the TB profile excluded unfortunately.

There were other suggestions then just excluding the Thunderbird profile folder.

I missed those. I'll get onto that right away.

Some sort of related things that might or might not help.

Run the disk cleanup utility that comes with Windows. (Windows does an appalling job of managing temp files. I have seen orphaned temp files crash a server let alone a desktop machine. Despite Microsoft's assurances that it does not matter how many files are in a folder, it does.) Ensure Wiindows defender is disabled. ((t get lots of bad press on this forum for some reason) Disable email scanning in AVG. It will buck. But the risks are so close to Zero I fail to understand why we have it. (Try it and see what happens)