IIS security

Hi,

On my website, running on IIS 6, I need to grant access to only user
accounts that are in the AD.

Some PCs that access the website are on the domain, others are not.

I have set up the website to use intergrated windows authentication.

For the PCs that are outwith the domain this is ok because it prompts
them for a user account and password.

However, for the PCs on the domain, that are logged in by valid user
account, it still prompts for a username and password.

How can I set up IIS to only prompt for username if they re not already
logged on with a valid account?

Thanks,

Gary.