If you are using Outlook 2007, you need to consider installing this fix: http://support.microsoft.com/?kbid=961752
I've had so many frustrations with Outlook 2007 in recent times, mostly performance issues and the dreaded messages about Outlook not having been shut down properly last time it was used. I've also had some serious issues with calendaring, etc.
This update makes a stunning change to the responsiveness and stability of Outlook. If you have any doubt, the hotfix above includes performance improvements that will come in SP2 and are documented here: http://support.microsoft.com/kb/968009/ But the hotfix above is the one to install.
Thank you to Alon Brown and all in the Outlook team for this work!