I am getting the following message when I try to access addresses and at other times.  I turned off Messenger and that helped eliminate the number of times the message comes up:

 

      Either there is no default mail client or the current mail client cannot fulfill the message request.

      Please run MS Outlook and set it as the default mail client.

 

I currently use and want to continue using Outlook Express as my mail tool.  I noticed however, the box to make it default is greyed-out.  What am I missing?  How do I eliminate the error message?