Task Scheduler not recognising user

I'm trying to shutdown a computer automatically every evening. I have programmed a shutdown item in the Task Scheduler but when it gets to the user information it doesn't recognise my user. The error message I get is "Unable to establish existence of the account specified" while I'm creating the scheduled task. I have tried making a new user but it doesn't help the issue. I have read other posts saying that xp has issues with user recognition and have not seen a successful resolution yet.
Does anyone know of a solution to this?
Problem Solved

We worked out that we needed to add a password to our username to make Task Scheduler work.

We then went to the START menu and chose RUN and typed: control userpasswords2 to be able to turn on automatic logon.

Tried to link to articles that helped but we haven't posted enough.