Kerberos_Error_ 7 Windows XP SP3 Active directory Server 2008 R2 SP1

Two Windows XP can't logon domain suddenly.

Any advise?

Thanks.

KerberosError 7

The kerberos subsystem encountered a PAC verification failure.  This indicates that the PAC from the client  in realm had a PAC which failed to verify or was modified.  Contact your system administrator.