iis guest account failure

Every morning our intranet web site asks for logon details although we have not set it to do this. The IIS guest logon account should logon to the site automatically. If we restart the machine the site stops requesting a logon. Looking at the event logs the computer tells us that the iis guest account does not have permission to log on, however this also stops after restart. The guests local group and the account is enabled. The registry shows the FTP service allows guset logon so why is this happening?

Any help would be appreciated.