Although you do not say what operating system you are using, I will assume that you are at least using Internet Explorer 6 (IE6) for a Web browser. If that is the case, then try adding the URL of your Exchange Server to the browser's Trusted Sites list. Find this in the IE properties under the Security tab.
Also, it would be helpful to know if the problem is user-specific (i.e., it follows certain users from computer to computer), or whether it is computer-specific (i.e., it happens to any user at a given computer, or only to a certain user at a certain computer).
MEMBER FEEDBACK TO THIS ASK THE EXPERT Q&A:
This is a known Internet Explorer 7 and OWA issue. My coworkers who installed Microsoft Vista have experienced this problem.
I found valuable troubleshooting tips in this discussion thread at Microsoft's TechNet forum: "Outlook Web Access -- unable to use reply"
Do you have comments on this Ask the Expert Q&A? Let us know.
Related information from SearchExchange.com:
Dig Deeper on Outlook management
Related Q&A from Bradley Dinerman
Find out how you can avoid DNS issues related to a Exchange 2003 server only collecting email from specific POP3 domains. Continue Reading
Are you having trouble with duplicate appointments in your Microsoft Outlook 2003 calendar? Learn what may cause this to happen. Continue Reading
Learn the best way to configure the default recipient policy in an Exchange 2003 test environment. Continue Reading