Make Outlook Web Access Your Default Mailer

1. Add the following to a .reg file (don’t forget to edit the location of the Exchange server)
   
Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail\OWA]
@="Outlook Web Access"

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail\OWA\shell\open\command]
@="\"C:\\Program Files\\Internet Explorer\\iexplore.exe\" http://server/exchange/alias/"
2. Import it into the registry by double-clicking the .reg file (Please know that mucking with the registry is potentially dangerous and can make your system unstable).
3. Boot IE
4. Go to IE’s Tools / Options… Programs tab
5. Choose “Outlook Web Access” from the E-mail dropdown.
6. Hit OK.

Source: http://weblogs.asp.net/tmeston/archive/2004/01/08/48837.aspx