I recommend trying the following:

  • (IIS6) Ensure the IIS_WPG group has access to your website root
  • (IIS5) Ensure the anonymous user has access to your website root
  • Disable MCMS on the site and access a page (might have to swap it to another one first)
  • Ensure NTAuthenticationProviders is set to "NTLM"
  • Ensure there are no invalid () users listed in Site Manager
  • Add a TraceSwitch named CmsAuthTraceSwitch with a value of "4" and a TextWriterTraceListener to output MCMS's inner thoughts