I have been using Iolo Antivirus (System Mechanic) for years but never had this before.
The problem is quite intermittent. Several times one day and then not again for a week or so. Guess I'll just have to persevere....
Some of my incoming emails have recently appeared as plain text with lots of coding after the message "this is a multipart message in mime format".
The problem appears random in that it does not always occur with any particular sender. Some emails appear OK, some are corrupted.