Outlook 2007 upgrade from 2003, using MS Exchange Server 2003

[Mihai]

I just installed the entire Office 2007 RTM, including Outlook 2007. When
installing, I choose to upgrade my existing 2003 installation, which worked
well for all the office components besides Outlook. When I try to start
Outlook, after trying to connect to the Exchange server for a while, throws
an error message and closes: "Cannot Start Microsoft Office Outlook. Cannot
open the Outlook window. The set of folders cannot be opened. The attempt to
log on to Microsoft Exchange failed."

Any ideas?

Thanks