Setting Default Mail Clients By Users Other Than Administrator?

Some of our systems got their default mail client reset to Outlook Express.Most of our users use Outlook, with a few using Thunderbird.It seems that only an Administrator can reset their default mail client through Internet Options.I came across article from Microsoft that says that only an Admin can change this after an admin sets it.I find if very hard to believe that our users cannot set their own default mail client after an admin sets it once. Does anyone know how to reset this so users can select their default clients