Microsoft Security Advisory (929433)

Vulnerability in Microsoft Word Could Allow Remote Code Execution 
Microsoft is investigating a new report of limited “zero-day” attacks using a vulnerability in Microsoft Word 2000, Microsoft Word 2002, Microsoft Office Word 2003, Microsoft Word Viewer 2003, Microsoft Word 2004 for Mac, and Microsoft Word 2004 v. X for Mac, as well as Microsoft Works 2004, 2005, and 2006.

In order for this attack to be carried out, a user must first open a malicious Word file attached to an e-mail or otherwise provided to them by an attacker.

As a best practice, users should always exercise extreme caution when opening unsolicited attachments from both known and unknown sources.
http://www.microsoft.com/technet/security/advisory/929433.mspx