The above query is a common one that we receive from our customers. Have a look at our support KB regarding this (specific to US English version only) http://support.microsoft.com/kb/2625547 . This article provides a comprehensive list of the most recent hotfix updates and service packs for Microsoft Outlook. Some Outlook features are dependent on Microsoft Word and Microsoft Office components. Therefore, you should also install the latest updates for the Word and Office core components.
The above article is applicable to Outlook 2003, Outlook 2007 and Outlook 2010. Also, the above article will be updated to reference new updates as they are released.
Note: You can visit the Update Center for Microsoft Office (http://technet.microsoft.com/en-us/office/ee748587.aspx) to find links to the most recent cumulative and public updates for Microsoft Office.