unless I misunderstand what you want to do, xl should do this automatically. By default, xl will create a mailto: link when it detects a "valid" email address in a cell. xl will turn the text blue and underline it to indicate a link. When you click on that cell, your mail tool Outlook should open using whatever profile you have specified as the default profile or ask which profile to use if you have told Outlook to do that. When Outlook opens, it should present a new email window with the email address form xl already filled in.

If this is not happening then you probably have this option turned off in xl. In xl2003, navigate to Tools | AutoCorrect Options | Auto Format As You Type and see if the "Replace as you type Internet and network paths with hyperlinks" box is checked.

Do you want the new email window to have more info than just the address?