If you recieve the following popup error upon starting Outlook, then you have a corrupt registry key that can easily be corrected.

This is documented in a Microsoft knowledgebase article:

http://support.microsoft.com/kb/823633

The short version is:

1. Close Outlook
2. Open the Registry Editor
3. Drill down to this registry key:

    HKEY_LOCAL_MACHINE\Software\Microsoft\Exchange\Client\Extensions

4. Delete the value named "Exchange Extensions"
5. Launch Outlook again. Outlook will recreate the key.