Changing email clients from Outlook Express to Outlook

I have been using Outlook Express as my default email client. I have
recently installed Outlook 2003 on my computer and am changing my default
email client to Outlook. Everything is working fine (composing, sending,
receiving, etc.) in Outlook. However, when I go to send, for example, a
worksheet from Excel I'm taken through the process of setting up an email
account - in Outlook Express. And the page is sent via Outlook Express
instead of Outlook. I can just open a new email to that person and attach
the sheet; it would be nice if I could figure out why it is taking me through
Outlook Express instead of Outlook when I've set up Outlook as my default
email and removed all of the account info from Outlook Express. Any
thoughts??