I have been tackling this problem for a while now. I have a single user in a 30+ user environment running AD and Exchange 2003 Server. The user is running XP with Outlook 2007. At least once every few days, when Outlook is launched by the user, nothing happens. When you look at task manager, there are at least 4-5 or more versions of Outlook.exe running. One of them is usually taking about 95MB of RAM or more. If you kill that process, the others go away and Outlook appears.
We thought it was user "eagerness" at first (click-happy) so we setup their shortcuts to use the /recycle parameter, but this is proving not to work. We have tried safe mode, repairing, looking for spyware/malware as well. We have removed and reinstalled. The event log doesn't show anything. I am out of ideas as to what is causing this to happen. No other users are experiencing this and nothing else is being reported as going awry on this machine. I know Outlook has it's bugs, but this is strange. Also, the user never had this problem with Outlook 2003. Thanks!
This happens to me sporadically, and it always seems to be a COM add-in that keeps Outlook from shutting down completely (CRM, in my case). The following is old:
http://support.microsoft.com/default.aspx/kb/914909
but describes the problem pretty much exactly as well as potential solutions (really just "find the bad add-in").