ASP app needs Anonymous and Integrated Windows Authentication but not working on server

Question

I have an ASP application that requires an authentication handler where it redirects users based on the AUTH_USER server variable.

I have IIS 5.1 on my PC and when enabling both Integrated Windows Authentication (IWA) and Anonymous this seems to work fine.

When I deploy to Windows Server 2K3 it all changes and with both IWA and Anonymous enabled it seems to default to anonymous.

Any ideas on how I can get around this? Essentially I want to pass trusted domain users to one site and non-domain users to another, it shouldn't be that hard!

Thanks