Authentication Failed Logons

I've been getting a lot of User Logon Failure alerts for a few systems on my network that are trying log in with a disabled guest account. What could be the cause of this problem?

EventType: UserLogonFailure

EventInfo: Account "Guest" used for logon failed from "XXXXX-XXX"

ProviderSID: Microsoft-Windows-Security-Auditing 4776

ExtraneousInfo: Error Code: 0xc0000072 Error: account is currently disabled

DestinationAccount: Guest


FailureReason: account is currently disabled