Hi Proff,
Sounds to me like default.aspx isn't set up as a default document in your IIS site and the 403 error is in response to trying to list the directory. Ensure you have ASP.NET and ASP.NET 2.0 installed (which you should have) and that they are registered in IIS. You can do this last bit by opening a command prompt and running:
c:\windows\microsoft.net\framework\v2.0.50727\aspnet_regiis.exe -i
This will ensure ASP.NET has it's appropriate script files etc setup. You should then ensure that default.aspx appears as a default document in your site - open IIS, right click your website or virtual directory and select properties. Then go to the documents tab and ensure default.aspx is there - if not, add it in.
Hope this helps,
Cheers,
Tony