do you use active directory? if so, here are some ways..
create a GPO that will force the computers to have the default email to be Lotus Notes
here are the basics:
http://support.microsoft.com/kb/555566here is how:
http://support.microsoft.com/kb/556081 and the more detailed one:
http://configuration-guide.com/microsoft-outlook-default-mail-client-group-policy/you may need to change it to "Lotus Notes" first, and if you need to revert it to outlook just do the way it said on the guide above
hope this helps!