Just thought I'd pass this along. I have been fighting with an ami's IIS for the last few days and after using http://www.microsoft.com/downloads/details.aspx?FamilyId=E90FE777-4A21-4066-BD22-B931F7572E9A&displaylang=en I found that the IUSR account IIS was expecting to authenticate anonymously was WRONG! For some reason the one on the machine was different than the machine name, I suspect this is because the machine name is generated when the AMI starts up. The way I fixed was to go into the My Computer > Manage > Local Users and Groups > Users and rename the IUSR account with the correct machine name then restart the IIS and everything worked!
Hopefully this saves some people time fighting with IIS.