Probably something in IIS itself that is not set properly.
Might start there by checking if you can browse a non-.Net file such as "403-3.gif", in the root of the website.
If this doesn't come up, then you might check DirectorySecurity tab in IIS and/or the host header values to make sure URL/IP you are using is pointing and setup correctly.
If the 403-3.gif does come up, then it might be IIS not working happily with the .net framework (which I find to be slightly-rarer situation on Win2k3).