I have noticed alot of these errors in the security log of one of my DCs (called DC1).

"The computer attempted to validate the credentials for an account.

Logon Account: None
Source Workstation: DC1
Error Code: 0xc0000064"

And every now and again the Domain\guest account gets locked out (even though its disabled). I unlock the account and within a minute it is locked out again.

I have tried running Network Monitor to capture where ever the log in request is from but I can't see anything thats causing it. It says the source is DC1 one but I assume that is because the log in request being made on that DC.

Any help would be appreciated.