On inspection I often find several instances of the Outlook.exe process running. Terminating them all fixes the problem.
This multiple-instance non-existing process problem is longstanding. It's easy to find reports for Outlook 2000 to 2007. Part of the problem is that Outlook's COM add-in infrastructure smells like raw sewage.
This Slipstick page gives a good overview of known causes and management options: Microsoft Outlook: Outlook.exe won't exit.
Other more or less useful references all illustrate how damned problematic Outlook is:
- http://support.microsoft.com/kb/954642/en-us: Microsoft’s own Office Communicator is a bad actor.
- http://support.microsoft.com/kb/948733/en-us: More Communicator problems. Mercifully, I killed that foul spawn.
- http://support.microsoft.com/kb/957909/en-us: The post-SP1 hotfix may help, but most of us will want to wait for Office 2007 SP2.
- From 2005, but still valid: Note in comments Outlook's iTunes add-in is a potential bad actor. I killed that one, but it didn't help. There are a lot of Add-In issues out there, I don't think it's possible to write a safe Outlook 2007 add-in.
Using File:Exit rather than Alt-F4 helps some people. Certainly everyone should be very cautious about installing any Add-Ins to Outlook -- not least Add-Ins authored by Microsoft.
Note that many corporate customers cannot turn off antiviral scanning of Outlook, a common problem that I suspect occurs when PST files get large (mine are GBs).
Rumor has it Office 2007 SP2 will fix these problems, but the Outlook shutdown and COM architecture problems have been around for over 9 years. They won't go away easily.
My cynical suspicion is that Microsoft will find they’re going to kill large pieces of their LiveMeeting/Sharepoint/Communicator platform by doing this. I’m also a bit skeptical of Ryan’s explanation; the problem is not only that the processes hang, it’s that on relaunch Outlook creates a new process rather than reconnecting to the running process.