Cannot compose or write email in Outlook Web Access on Vista

check out these links -

fix for exchange server:
http://www.mydigitallife.info/2006/12/09/cannot-compose-or-reply-to-messages-and-other-errors-in-outlook-web-access-in-windows-vista/

fix for IE7:
http://mustech.net/2007/02/20/windows-vista-webmail-owa-fix-for-internet-explorer-7-five-step-fix

The IE7 fix worked just fine for me. Haven't tried the exchange server fix, since it requires to bring down the server.