"Send to" mail recipient opens wrong email account

When I right click on a file in My Computer or Windows
Explorer and select the Send To Mail Recipient, XP
attempts to send the file via an MSN account even though
my default email program is Outlook. I have the identical
accounts on my Windows 98 machine, and the "send to mail
recipient" feature works perfectly (it opens an Outlook
new message with the selected file attached). How can I
get XP to do the same?

Thanks very much for any help.