Security Audits - 2000 Server

I'm trying to learn how to read the Security Audits in Event Viewer on a SBS2000 server.
I need to know more about some messages I see often, and what I should be doing to resolve them. Here are the ones I need.

1. This one constantly shows up from different IPs...
---------------------------
Category: Account Logon
Event: 677
Type: Failure Audit

Service Ticket Request Failed:
       User Name:      
       User Domain:      
       Service Name:      krbtgt/OVATION.LOCAL
       Ticket Options:      0x2
       Failure Code:      0x20
       Client Address:      192.168.40.79


2. This one happens a lot as well...
--------------------
Category: Account Logon
Event: 675
Type: Failure Audit

Pre-authentication failed:
       User Name:            jsmith
       User ID:             OVATION\jsmith
       Service Name:            krbtgt/OVATION
       Pre-Authentication Type:      0x2
       Failure Code:            0x18
       Client Address:            192.168.40.79

^^ those are the main two that constantly show up...if I can get some help on exactly what's going on and how I can resolve the issue, I'll then post some more events in another question...thanks!