Outlook 2000 will not quit in Windows XP

I recently replaced our Windows NT servers with new Windows 2003 domain controllers and migrated Exchange 5.5 to Exchange 2003. I also upgraded many of our PCs to newer systems with Windows XP. The PCs are running Outlook 2000. Since all the upgrades, Outlook will not quit all the way out on the Windows XP computers. The only way to close it is to use Task Manager. Any suggestions?