Shgoh is right. Please note however that previewing is the same as opening the email to your email client. That means that webbugs may be activated. Webbugs are not viruses but something that indicates to the sender that your email has been received and viewed, this is especially useful to spammers so that they know to send you more spam.

So even with avast! or any virus scanner I still prefer not to use the preview pane. You never know what kind of garbage may pop up in it.

If you must preview messages I would use a program such as MailWasher that can view messages on the server without activating webbugs, viruses, and such.