Force Default E-mail Client ??

We are upgrading our mail clients to Outlook in a few weeks, but for now we use Lotus Notes.   The Outlook client has been silently installed on all the machines in the building.  Though when some users  click on a online link (contact us, for more info etc) they are prompted to configure Outlook for the first time.

Does anyone know of a way were we can force Windows to set Lotus Notes as the default e-mail client and calendar and not change it? Using a registry key perhaps?  

But the gotcha is in a few weeks  we will need to awaken outlook then set permanently set Outlook as the default e-mail client.

Anyway to do this?