Outlook error message

[PUR]

I have a new installation of Office 2007. My e-mail is connected and
sending/receiving messages. But I keep getting this error message: Either
there is no default mail client or the current mail client cannot fulfill the
messaging request. Please run MS Office Outlook and set it as the default
mail client."

Outlook IS set as the default mail client. Why am I getting this error and
how do I make it stop?

Thanks, PUR