After trying to debug an addin for Outlook 2003 with no joy I turned to google for support, this on previous attempts had not come up with any good solutions but I managed to find the following post: http://www.analogousmember.com/archives/000394.html that resolves the solution.
It appears that outlook will try to load up the 2.0 framework when debugging thus would not allow .NET 2003 to debug it. The solution is to create an Outlook.exe.config and force it to use a 1.0 or 1.1 framework and then everything is fine, now I can get on with the job in hand.
<configuration>
<startup>
<supportedRuntime version="v1.1.4322"/>
<supportedRuntime version="v1.0.3705"/>
</startup>
</configuration>