If you click on the FROM button when you're responding to a message does it list your default e-mail account? What kind of account is this? POP3? IMAP? Hotmail? Exchange? Something else? Does this happen even if you start Outlook in Safe Mode? (hold down CTRL while you click the icon) See if this helps:
http://www.officeforlawyers.com/outlook/tsol.htm#safe