Hm, well that makes it harder then. Have you already attempted to uninstall and reinstall CenturyLink?
Option two is to use an email file... right-click it, select "Open With", then select "Choose Default Program". Make sure the "Always use..." box is checked. If you do not see your email program listed, then manually browse to the program's exe file.
If that doesn't work, then you should look within the program's own settings as most programs give the user the ability to make it the system default application somewhere in their own settings area.