We often heard about security problems of Microsoft Outlook (and Outlook Express).  If you don't have the choice to use another messaging software than Outlook or you don't want to install unknown efficiency patches, you can anyway simply protect yourseft against a certain type of virus.
 

Solution

Some types of viruses tries to spread themselves by sending a message to each of your contacts, one after the other.  The trick is to create a contact named !0000 without setting an e-mail address.  This contact will be placed first on your contact list due to the automatic sort in alphabetic order (and will be the first "victim" of a possible virus).  Si a virus try to send e-mails, an error will appear warning you for a messange has not been sent.  Cancel then delete the unsent message.

Another solution is to avoid using the Outlook contacts directory...  it's yours to choose!