Geraint,
What you are seeing is Microsoft's additional security implementation to
retard or prevent email viruses which target Outlook. Unless you are
working in an Exchange environment, this warning cannot be turned off.
If you simply want something that will automatically click the 'Yes' button
from within a program, there is a very nice, FREE utility available from:
http://www.express-soft.com/mailmate/clickyes.html
that will take care of that problem for you. It will require just a couple
lines of additional code (samples provided at website). I have used it in
client applications and have found that it works very well.
If you need more, the following is a link to an article by Sue Mosher
(Outlook MVP) which discusses a product named Redemption, an add-in which
provides a work-around to the security warning:
http://www.winnetmag.com/Articles/Index.cfm?ArticleID=24411
hth,