I have the very same problem and it's with the basic Outlook toolbars. If I
move the delete button down within the same toolbar real estate this happens
very quickly after customization. Now I have two computers, a desktop
running XP Pro and and laptop with a native install of Vista. In both cases
I'm using Office 2007/Outlook 2007. I do NOT have this problem in my Vista
install of Outlook 2007, but I DO with my XP install of Outlook 2007. Again,
it is NOT caused by a third party toolbar. It's a problem with the native
Outlook toolbars.
But then Outlook 2007 almost never shuts down adequately, either. My take
is that it's a buggy program and I hope these issues are being addressed for
an upcoming service pack.
Elliot Berlin