Getting "Error: Access is denied"

Hi
I would appreciate any help resolving this error I get at a client site:
When I browse to my ASP.NET 3.5 web application I get presented a login dialog box. It won't accept any valid credentials I input into this dialog box, the only option is to cancel. I then get presented with the following message: "Error: Access is denied" in the browser window. There is no additional information, just this unformatted message.
My Environment:
Windows Server 2003 R2
IIS6.0
ASP.NET 3.5 SP1
Two AD domains. Users and groups are in one domain, servers are in the second. I think this may be the issue, although I don't know how to get around it.
My Application:
A web application to administer BizTalk monitoring metrics and to provide a monitoring dashboard.
Connects to SQL using SQL auth, so this is not a problem.
Web application uses integrated windows authen ...