Security Success Audit - Event ID 680

Thanks for your help McKnife, the Client PCs are XPSP2 and DC is win server 2003

Thanks for your help also richrumple, i have a better understanding of this now.
I'm trying to figure out why why no reply came from the DC via the kerberos protocol (with the help of your last link) - any further ideas?

It seems however that when this event (680) occurs, the users have left the computer 'locked' instead of logged off - this appears to be a factor.
We do have group policy and wsus set up and now, scheduled tasks (the problem was present before we set up the tasks)
Any other reasons Why these account logon auths above be happening out of hours? I did see one event id 612 (Audit Policy change) on a client PC out of hours so, Would all of this be just because of an automatic gpupdate? Although the times do not match up.

To answer your first question:
On the security logs on the server, there are Success audits before and afterwards for many machines on the network
(Event IDs 673 and 674 as shown below)
The user on this is SYSTEM and not the users login ID so i wasn't too worried about this at the time.
__________
EventID: 673
Category: Account Logon
User: NT Authority\SYSTEM
Computer: SNN01

Service Ticket Request:
       User Name:            SNN06$@SNN.LOC
       User Domain:            SNN.LOC
       Service Name:            SNN01$
       Service ID:            SNN\SNN01$
       Ticket Options:            0x40810000
       Ticket Encryption Type:      0x17
       Client Address:            192.168.1.216
       Failure Code:            -
       Logon GUID:            {448f8589-4940-4d55-70c5-63ff742de829}
       Transited Services:      -
__________
EventID: 674
Category: Account Logon
User: NT Authority\SYSTEM
Computer: SNN01

Service Ticket Renewed:
       User Name:      MARKETING02$@SNN.LOC
       User Domain:      SNN.LOC
       Service Name:      krbtgt
       Service ID:      SNN\krbtgt
       Ticket Options:      0x2
       Ticket Encryption Type:      0x17
       Client Address:      192.168.1.48
__________
While we're here, are these particular logons kerberos logons so?


Also, this may not be related but within a minute after event 680 on the server, there are Application and System events on the client PC itself:
App error: event 1030 (Windows cannot query for the list of Group Policy objects. A nessage that describes the reason for this was previously logged by the policy engine). I looked back and saw event 1058 (Amoungst others) that suggested that a file (gpt.ini) in the Default Domain Policy folder could not be accessed.
Sys warn: event 40961 The Security System could not establish a secured connection with the server ldap/SNN01.snn.loc@snn.loc. No authentication protocol was available.

Sorry for the long winded reply!

Thanks again in Advance,
windylad