What is digital ID and Why can't I Reply?

Using XP and Outlook Express.

Received e-mail and when I tried to reply by clicking on OE Reply button, I got a message that said:
You cannot send message because you do not have a digital ID.
I then clicked on the button that said:
Get Digital ID.
I then got a This page is not available message.
What in the heck is digital ID?
Should I want it?
Why can't I click on Reply and just reply to the message I received?

aob

KB 168726 explains how to set one up; to get rid of it might be as simple as Tools | Accounts | Mail | Properties | Security and unchecking the Digital ID option.