If you use Internet Explorer's Tools / Internet Options / Programs tab, and toggle the E-mail drop-down list, you can see that Microsoft enables you to set Forte Agent, Hotmail, Microsoft Outlook, or Outlook Express as the default e-mail client for mailto links, MAPI calls, and Internet Explorers Tools / Mail and News e-mail links. These choices are located in the registry, as sub-keys of HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail.

When you select the e-mail client, the values at the HKEY_CLASSES_ROOT\mailto key and sub-keys are also modified, which means that all users will have the same default.