Windows Mail stopped showing message body

Windows Mail suddenly stopped showing messages in the message preview pane, in the open message window and in the compose message window. Headers display normally. As far as we can tell, everything else is working properly. The computer was turned on but idle. Mail worked properly until my wife went to bed. The next morning, she could not see the body of any message. Nothing was changed (by us) between the last time it worked and when it stopped. Nonetheless, I restored to a point a couple of days prior, to no avail. We had a similar problem with OE6 on one of our XP machines, which was never resolved.

OS is Vista Home Premium, on an Acer Aspire x1200 with 4GB RAM

Presumably you have got Preview Pane enabled.

View > Layout > Show Preview Pane.

Yes, preview pane is enabled. We changed nothing between when it worked properly and when it didn't. The message header information shows in the preview pane, but the body does not. Opening the message in a new window yields the same result. Opening a compose window to reply, forward or write new, again shows everything except the body. We can forward the messages or send an attachment as a new message, but we can't read, edit or write anything except the header info.

Virus protection is AVG 8.5.392 Free License with email scanning disabled.