First, here is a Technet article about default permissions in Exchange 5.5:
http://support.microsoft.com/default.aspx?scid=kb;en-us;168753
(please remove any spaces)
This next article lists the default NTFS file-level permissions. It might be useful also:
http://support.microsoft.com/default.aspx?scid=kb;en-us;147376
Now, the whole BUILTIN thing is just what Windows calls the groups that are made when Windows is set up:
ADMINISTRATORS
USERS
GUESTS ACCOUNT OPERATORS
SERVER OPERATORS
PRINT OPERATORS
BACKUP OPERATORS
REPLICATOR
All of these groups can be called either YOURDOMAIN\groupname OR BUILTIN\groupname. You will see both methods used if you look around. Yes, it is annoying, but that is how it works! So, don't be afraid if you see the BUILTIN used.
Now, I don't have an Exchange server to check, so I can't verify if the Guest account should have rights on a Site. I would think not, though. Unless you are running a mailbox for the Guest account, I don't see why Guest should have any rights. I would hope that your Guest login would be disabled, so why give Guest rights at all? But remember, I am not certain on this. I am not in an Exchange environment, soI don't remember its security settings anymore.
And as for the BUILTIN\BUILTIN account, that one is also strange. I want to say that I have seen that before also, but I can't say that for certain. Is your Administrator account renamed to be "BUILTIN"? If so, then that would explain it.
Probably this one is ok, though.
hope this helps
