Not all Outlook add-ins are programmed to cooperate with, or share internal Outlook objects.  Conflicts usually present themselves as certain add-in features not working, but may also result in Outlook crashing.  The best method to diagnose a conflict is disable all add-ins, and then enable the add-ins one at a time (just those you actually use) until the conflict presents itself or is resolved. It’s always best to uninstall any add-ins you are not using (many programs, including iTunes, will install Outlook add-ins for no apparent reason).

To Manage Add-ins for Outlook 2007:
Use the Outlook menu Tools > Trust Center > Add-ins (left side menu) > Manage: [ COM Add-ins ] GO (click the Go button at the bottom) and click the checkbox to enable/disable add-ins, then click OK.

To Manage Add-ins for Outlook 2003:
Use the Outlook menu Tools > Options > Other > Advanced > COM Add-ins (button at the bottom of the page) and click the checkbox to enable/disable add-ins, then click OK