If you have the Preview Pane enabled (as it is by default) in Outlook Express, you may be at risk. Any time you select a message (highlite it by single clicking) in the main pane, you are in essence opening the message. These new--and some not so new--HTML and other baddies could now be activated. And the PP can cause further complications. See this section of Microsoft Security Bulletin MS04-018 which patches one of these vulnerabilities:
If a user is running Outlook Express and receives a specially crafted e-mail message, Outlook Express would fail. If the preview pane is enabled, the user would have to manually remove the message, and then restart Outlook Express to resume functionality.
The Preview Pane is pretty much a useless feature anyway, so it just makes sense to get rid of it. Pretty easy to do also:
1. Open Outlook Express
2. Click View
3. Click Layout in the drop down menu
4. Click to tke the checkmark out of the box next to "Show preview pane"
5. Click OK and you're done!
Odds are you won't get infected in this way except by accident. And it would be safer to receive your email in plain text. But better safe than sorry.