Windows Mail scrambles messages

Lately, many of the e-mail messages I receive have been scrambled into
unreadability by Windows Mail, even though they were received in readable
form at my ISP, Comcast. These seem mostly to be messages that have somehow
been classified as junk, correctly or incorrectly. If I go to the ISP mail
files, I can see them, but when they reach Windows Mail on my computer,
they're gibberish. How dioes this happen? Cani I reset something so it does
not happen?
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kreinerh