Not being able to logon I was unable to diagnose the problem locally so I used the computer management console on another client to view the event log on the Exchange server. A quick look revealed a frequent error in the application log:

Event ID: 9157
Description: Microsoft Exchange System Attendant does not have sufficient rights to read Exchange configuration objects in Active Directory

The problem occurred because there are 3 security groups in Active Directory that Exchange queries in the Users Organisational Unit (OU) during startup:

Exchange Enterprise Servers
Exchange Services
Exchange Domain Servers

As it turns out, during a recent restructure of Active Directory someone had moved the above groups into a different OU and this was the cause of Exchange hanging and recording the above entry to the event log. Moving the 3 security groups back into the Users OU almost immediately allowed Exchange to display the winlogon screen without requiring a restart.

More information is available here:

KB910413

There is a known problem with Vista clients that causes IE7 to crash when using OWA. I have found this is usually when users have gone through the effort of composing an email and hit the 'Send' button. IE7 then crashes with the standard issue 'Program not responding' response at any attempts to recover from it.

I have only ever noticed this problem on Exchange 2003 SP2 and cannot confirm if it happens pre SP2 but it does only seems to affect Vista clients running IE7 and not XP running IE7.

The cure to this problem is a Hotfix released from Microsoft. Although the Hotfix has been out for a while, unless you have tried it by chance there is likelihood that you wouldn't as the description is somewhat vague. The knowledge base article can be found here:

KB911829

And you can download the Hotfix from here:

Download Hotfix for KB911829

Install the Hotfix on your Exchange OWA facing server. You will have to reboot your Exchange server and key services like 'Microsoft Exchange Information Store' are stopped during the installation. This will obviously cause all outlook clients to disconnect from the Exchange server. You may also then need to instruct Vista users to log into OWA and navigate through:

Options -> E-mail Security -> Install S/MIME

to reinstall S/MIME.