Are your system configurated to Heuristics analysis (Internet Mail Provider)?
avast! can check the HTML part of a message (body). Some viruses make use of bugs in some mail programs (especially unsecured MS Outlook and Outlook Express) that make it possible to start the virus merely by viewing the message in preview pane. avast! checks whether the HTML code of the message does not contain a tag enabling such a trick. If yes, the warning message will be displayed.
Take care! See you round… 