Outlook 2007 Integration

We have a written a C# application that interfaces with Outlook using
the API.

Since Microsoft Office 2000 SP1, our users have had to put up with the
very annoying warning popup about a program accessing Outlook and do
they want to allow/deny.

However, since migrating many of our users to Office 2007, some (not
all) of our Office 2007 users no longer get this warning message and
we would like to know why this is the case so that we can do the same
for the others to stop them getting the warning (like it was before
Office 2000 SP1). It is nothing that they remember setting in the
Outlook configuration.

It is definitely not an application such as ClickYes running as I have
checked and nothing definitely pops up when sending emails.

The users who do not get the popup are running on new Dell equipment
and I presume that they only installed Office 2007 on their new
environment whereas the others would probably have upgraded from
Office 2003. Could this be the answer?