Outlook 2007 crashes with Windows 7.

Outlook 7 installed with SP2.
Recently upgraded Vista to Windows 7 Ultimate 64 bit.
Outlook 2007 ran without incident.
Installed Microsoft's Security Essentials. Office 2007 ran.
Several days ago I ran Outlook with no problem. Fifteen minutes later I
tried to run Outlook and the opening window flashed on the screen and
disappeared and the error message of
"Cannot start Microsoft Outlook. Cannot open the Outlook window".
Ran the Diagnostic Tools with no result.
This does not appear to be the fault of Outlook but something else that is
blocking the opening of Outlook.

Any ideas available of what to try?

Thanks