Go to IE>Tools>Options>Programs. There are dropdown-boxes there that should offer you the choice of all email-programs present on your machine. Just select the one you like.
Hope this helps.
Kees
Somehow I accidentally said yes the last time Outlook asked if I wanted it to be my default. I followed these directions to undo what I did:
On the Tools menu, click Options, and then click Other. Under General, select the Make Outlook the default program for E-mail, Contacts, and Calendar check box
Problem is - I don't have the option to deselect the option.....what am I missing? Any suggestions?!?! TIA

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic