If you've installed or uninstalled Outlook 97/98/2000/2002/2003 since you
last installed Outlook Express, the process tends to break the Outlook
Express registry settings.  This can prevent you from selecting Outlook
Express as the default mail program, or prevent the selection from sticking.

- First, go to Start, Run and enter the following (with the quotation
marks):

"C:\Program Files\Outlook Express\msimn.exe" /reg

This assumes you have the standard path to msimn.exe on your PC.  If you get
a "File Not Found" error, use 'Start|Find|Files or Folders' to determine the
correct path.

http://support.microsoft.com/?kbid=306098
OLEXP: "Send To" and E-mail Links Do Not Work Using Outlook Express as
Default

- If the above doesn't work, see the WIN.INI MAPI fix here:

http://www.okinfoweb.com/moe/mail/mail_022.htm

- If you still have problems, then In My Computer, click Options (or Folder
Options) on the View menu, and then click File Types. Click the
URL:MailToProtocol line to select it, click Edit, click Open in the Actions
box, and then click Edit. Click the "Application used to perform action" and
make sure that it has the following:

"C:\Program Files\Outlook Express\Msimn.exe" /mailurl:%1

- For OE5 and above, try this KB article:

Fixmapi.exe Tool Included with Internet Explorer 5
http://support.microsoft.com/?kbid=228457

Hal