Sending attachments via email inOutlook 2000 can sometimes cause mail recipients to receive those attachments as a Transport Neutral Encapsulation Format (TNEF). The Winmail.dat file is used for this spec. If you want to prevent this you can go into the settings and set your mail client to send either HTML or Plain Text. setting your client to use RTF (Rich Text Format) seems to be related to the problem.
http://support.microsoft.com/default.aspx?scid=KB;en-us;q241538