I have one computer in the office running Outlook 2010 that stopped connecting to my Exchange 2003 server. I tried deleting the Outlook profile and creating a new one, but even with the new profile I receive he message "Cannot open your default e-mail folders. Microsoft Exchange is not available. Either there are network problems or the Exchange server is down for maintenance." I tried setting up a different user and I get the same result so the problem is definitely with Outlook on this machine.
There are no network issues on this machine and the Exchange server is not down.
I tried to repair Office and I get the same result. I tried turning off encryption and nothing changes. The mailbox(es) can be accessed via OWA without any issues.