Let me see if I have this right:
Windows Mail will let you read ROT13 text in someone else's NNTP newsgroup post if you open the message; there's an Unscramble option available at that point in the Message menu.
Heaven forfend, however, that you should want to create ROT13 text in a message of your own, or (gasp!) to read ROT13 text that someone has inconveniently posted to a blog out on the Web somewhere, because there seems to be no comparable command anywhere in Windows Mail or Internet Exploder for generating ROT13 text.
Now, given that in fact several someones have posted ROT13 scrambled text in a blog that I occasionally frequent, what is a rational blog-reader supposed to do?