Our admins use the built-in C$, D$, etc shares in Windows Server 2003 to access these disk drives from the network as authenticated domain admins. As soon as last night, we were able to use a Windows XP client and connect to the \server\C$. Our admin was prompted for credentials and then the directory contents were listed in Windows Explorer. This morning, we tried the same thing, and no credentials are prompted for. Instead the XP client just goes to an empty Windows Explorer window. This behavior is confirmed on a second XP client.
In short, what we are seeing is that we are trying to go to \server\C$, not prompted for credentials as we feel we should be, and then brought to an empty page.