Have you rebooted your computer since this has happened? Never underestimate the problems that magically get solved by rebooting.

Disable e-mail scanning in your antivirus program.

It is recommended that e-mail scanning be disabled in antivirus programs because antivirus programs already automatically scan everything when it is accessed. So e-mail scanning is an unnecessary duplication, and e-mail scanning itself can cause problems.

If there is still a problem after rebooting and disabling e-mail scanning in your antivirus, then:

1. Run a Check Disk to fix any file system errors. <a href="http://brucejohnson.ca/CheckDisk.html">How to run a "Check Disk" in Windows</a>

2. Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled".

If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Thanks Bruce - tried all your suggestions but no useful effects. The checkdisk info was interesting - I haven't done checkdisk for a long while! I will also get on to my antivirus (ESET) and see if they can shed any light. Luckily my garbled inbox contained less important emails which hadn't been filed elsewhere. New incoming emails are fine.

Since it is just old e-mails that are corrupted, then data recovery is in order. Specifically, looking at the mbox files and compare the good e-mails to the corrupted to see the difference. There may be one specific thing that has gone wrong with each e-mail that can be fixed with a search and replace. Or it may be more complicated than that.

I recommend backing up the Profile folder (which should be done regularly anyway). Then either you, or someone more knowledgeable about mbox files, figure out what is wrong with those e-mails and hopefully fix them.