HTTP 403.x-Forbidden (IIS 6.0)
Typically, 403 errors occur when an operation or request is disallowed because a requirement other than proper authentication credentials is not met. Commonly, this error occurs when a script request is made to a Web directory for which script access is not enabled. In such a case, first verify for which resource the request is being made. If the request is, indeed, for script code, such as ASP or ASP.NET, check the execute permissions for that directory in IIS Manager and ensure that at least Script is selected.
Table 11.7 HTTP 403 Substatus Codes lists the HTTP 403 error substatus codes.