Hotfix required to get Exchange OWA to work on Vista
Vista removed some DHTML support that was used in Exchange 2000/2003 OWA. To get it to work correctly you need to update all your exchange servers (frontend and backend) with KB 911829 (https://support.microsoft.com/kb/911829).
Once you install this update you will be able to do more with OWA and IE7.
Enjoy