My GPO for Interactive logon: "Message text for users attempting to log on" applies to my servers but not to client OS (XP) within my network.

Apply the GPO for interactive Logon to Default Domain policy on Top level so that it will apply to all OU

Already done so. I did this on my primary domain server. It works when attempting to log onto servers only, but will not apply to client users on XP systems.

Great!! Please check on client XP machine using GPRESULT /V  and see the policy is reflecting or not.

Also you can try to gpresult/force to replicate the policy

already tried gpresult/force to no avail, still does not apply to XP systems. XP systems are all joined to my domain.

WHat is your Domain - WINDOWS 2003 or Windows 2008 ?

Refer this .same question has been asked - https://www.experts-exchange.com/questions/26266304/interactive-logon-win-2008-r2-windows-xp.html

using windows server 2003.

If you applied correctly the Policy for interactive Logon it should reflect to XP machine.
Run RSOP and check its reflecting under this or not.

Have you applied any permission restriction.. Ideally Authenticated user should have permission.

I advice you to recheck your domain policy/Permission and client side: RSOP/Gpresult

Where this GPO is linked in the domain?

can someone show me the correct method for applying the GPO for this to work? I first applied it on my primary domain controller, seems to work fine when authenticating to servers, but not XP systems. How should I apply GPO to the XP systems to get it to work?

Just 1 question - How Windows XP allowed to loging to domain,  if DNS pointing to ISP domain ?