Outlook problem

Several of my users cannot enter text in the body of e-mail messages. Removing the signature file solved the problem for some of the users some of the time, but it still exists. (The cursor disappears when you click in the body of the e-mail, however, you can enter subject and address info)
This is Outlook 2000 on Exchange, if that matters. I will award mad points to the person who solves this for me.