Launching virus from preview pane

Yes, it is possible. "Messages exploiting certain vulnerabilities in HTML mail can force a malicious file attachment to run even if the user only views it in the preview pane or opens the message."

For more info, see:

www.slipstick.com/outlook/antivirus.htm

Slipstick.com is a great resource for Outlook and Exchange users!

You need to apply the patch provided by Microsoft to prevent Nimda and other similar viruses from launching themselves simply by previewing an email.
See the following page on Microsoft's Technet:

http://www.microsoft.com/technet/treeview/default.asp?url=/technet/security/bulletin/MS01-020.asp

Go to any Anti Virus site NAI,Trend,Sophos etc and look up the characteristics for the KAK worm, This is one that runs when previewed. There are others, mainly the Javascript or VBScript types where all that is needed is the Scripting host to be present and unsecured.
There are many security patches available, Some will cause more hassle than the very thing they are supposed to prevent so a testing regime and deployment of only the essential and least harmful patches will save you lots of calls from your clients (users)
(I keep being told by my directors not to call them users as it sounds derogatory, and I thought users was polite considering the names they are usually called!)

as with other answers check MS updates and KB and also visit www.slipstick.com for some
impartial advice and probably the best Exchange\Outlook site on the Internet