The Mysterious ADVAPI logon failure

Hello,
Found something that is disturbing (to say the least) on my server 2008 domain controller event log. Both of which occured today almost exactly 7 hours apart:

Here is the log entry:

An account failed to log on.

Subject:
      Security ID:            CORPHQ\administrator
      Account Name:            administrator
      Account Domain:            CORPHQ
      Logon ID:            0x3307198

Logon Type:                  3

Account For Which Logon Failed:
      Security ID:            NULL SID
      Account Name:            Guest
      Account Domain:            CORPHQ

Failure Information:
      Failure Reason:            Account currently disabled.
      Status:                  0xc000006e
      Sub Status:            0xc0000072

Process Information:
      Caller Process ID:      0x13dc
      Caller Process Name:      C:\Windows\explorer.exe

Network Information:
      Workstation Name:      SERVER (This is not the real name, for security reasons)
      Source Network Address:      -
      Source Port:            -

Detailed Authentication Information:
      Logon Process:            Advapi  
      Authentication Package:      Negotiate
      Transited Services:      -
      Package Name (NTLM only):      -
      Key Length:            0

All values are accurate, except for the server name, which I changed for security reasons. Does anyone know anything about this? I've not seen this in the event log before. What's more bizarre is that it's trying to use the guest account. Can anyone provide insight as to what the Advapi Logon Process is? Obviously my server recognizes it?