There are 3 ways to fix the issue:
1. Use another browser , e.g. Chrome ( most preferable one :-) )
2. Fix your browser. This is not recommended (you don't want to fix everybody's browser do you?)
Use it at your own risk - I haven't tested it.
Here are some explanations:
Here is another fix , but it only works with Exchange 2003 OWA (Exchange 2000 doesn't have "Email security" under "Options")http://mustech.net/2007/02/20/windows-vista-webmail-owa-fix-for-internet-explorer-7-five-step-fix
3. Fix your Exchange server.
The hotfix itself never appears on Windows/Microsoft Update - this is not a bug, this is a feature, right :-)?
You can download the hotfix for Exchange 2003, but if you have Exchange 2000 then you can call MS back for support and they gladly provide you with the hotfix for Exchange 2000.... for additional cost of course. Be prepared to pay at least $300 upfront. You can also download it from 3-rd party at your own risk here: