Error 645 Internal Authentication Error
This problem tends to occur between the customers modem and the phone line
 

• This error can occur if the Require encrypted password option is enabled on the Server Types tab in the connection's properties, or an incorrect user name or password have been entered
   
• This error has been known to occur when setting up a Virtual Private Network (VPN) connection in Win 98/ME. Check this by going into Internet Properties and then selecting the Connections tab and then verifying that the settings are correct (see Connection Settings for reference)
   
• Recreate the Dial up Connection
   
• Ensure that Dial-up Adaptor exists in the Network Components available in the Network settings accessible in the Control Panel
   
• If this does not resolve the problem reinstall the modem (Reinstall)