Can only get access via Guest Account

I have small, home network utilising XP Professional.

One machine functions as a printer server. Until
recently, other machine have been happy to connect to
this machine using Administrator and password when
trying to connect to this machine, from other machines,
for printing, and when challenged. Now I find, this
machine is refusing even the Administrator username and
password, and will only allow remote access when the
Guest account is enabled. I have made no changes to the
ACL or any other configuration changes. Any ideas?

Allistair Lomax