Occasionally people will see weird html text in their messages and think Outlook (or Word) is really screwed up. A few people think they are infected with a virus. If the text uses brackets like this: {HYPERLINK} it's a field code. Press Alt+F9 to toggle it the field and show or hide the code.

If the message contains real HTML tags, like <a href=, then it means an HTML message is malformed and Outlook is treating it as plain text, not HTML formatted. In most cases, it's a problem with the sending mail client but it also could be caused by either the sender's mail server or yours.