This is something I’ve been dealing with for a few years, but I have received a number of questions about it recently. Microsoft Exchange 2000/2003 uses an ActiveX control for the message body (and other features) in Outlook Web Access preventing many users with newer machines from being able to use OWA very effectively. The problem is not newer hardware, but an update for Internet Explorer that changes how certain ActiveX controls are used. This update was released in December of 2007, and is included in IE7 and IE8. When you try to create a new message (or reply/forward an existing one), you see something similar to this:
The Exchange Team wrote a blog entry about this, explaining it in further detail. The end fix is to install this update for Exchange 2003 (or contact Microsoft support for the Exchange 2000 hotfix), which changes the editor in OWA to use an iframe instead of an ActiveX control. It will require restarting a number of services (IIS Admin, WWW, Information Store and more), but will not require a server reboot.