In IIS6, under Web Service Extensions, make sure you have ASP.NET V2 Allowed, and Active Server Pages Allowed.   On the website properties, ASP.NET should be version 2 instead of 1.    See if there are any errors in the server event logs.    Network Service should have full control over the folder, and make sure permissions are inherited by sub folders.  Can't think of other items at the moment.

Another thing to check:  make sure that default.aspx is set as a default document under the Documents tab.  This has been a consistent issue for me for several months now... I think there was a security update a couple of months that when you create a new website, it no longer automatically assigns default.aspx as a default document. Not sure why.