When using Internet Explorer, my users can't see the reply field in OWA and instead see this:
Anyone have any idea what could cause this?
You include little detail, but hopefully my psychic powers are working here.
That looks a lot like you're using Exchange 2003 and Windows Vista or later as a client. Exchange 2003 used to use an ActiveX control for the editor, but Windows Vista and later do not include this ActiveX control, so they released a hotfix to change the behaviour. When I installed this on my Exchange server some years ago now, it required a reboot - your mileage may vary.
As a side note, I'm pretty sure they will have released this patch on Windows Update by now - when did you last update this box? If you really haven't done Windows Updates since this patch was released, for your own good please get installing those patches as there's guaranteed to be some critical security patches in there.