Maybe I'm misunderstanding you.

The default profile is the one that the user is going to use - normally you only have one profile active and that id the default profile.

Within each profile there are a number of services, one of which will be exchange.

I'm also confused by an earlier statement:
Quote:

I have tried to solve this with the registry, but opposed to my hopes, the HKEY_CURRENT_USER Entry (HKCU\Software\Microsoft\Windows NT\Windows Messaging Subsystem\Profiles) has the profiles of every user in there


This is not correct. HKCU will only have the profiles of the current logged in user.

Is it the profile that you are trying to find, or the service?