Reset AD\IUSR_%computername% password to match the stored password for this account in the IIS Metabase. If you are using anonymous authentication, it's likely that your IUSR account is out of sync with the metabase and AD. You will need to download MBExplorer from the IIS resource kit tools. Use that to get the password for the IUSR account. Then reset the password in AD.
Applies To Windows 2003 SP1 Notification Server 6.0 SP2