If you get the following popup notification when attempting to send a fax from Office 2003, scroll down to see the solution.
The problem is that even though you may have Outlook 2003 installed and even though you have activated it to work with InterFAX, Outlook is not configured as your default email program.
Here's how to turn Outlook into your default email program:
From within Internet Explorer click Tools -> Internet Options (alternatively, from the Control Panel, click Internet Options), then select the "Programs" tab and select "Microsoft Office Outlook" as your email client. Click "OK".