You find that the %SystemRoot%\Debug\UserMode\Userenv.log file on Windows XP contains the CheckXForestLogon: policy set to disable XForest check string in an entry.

This behavior occurs when you use verbose USERENV logging to troubleshoot GPO application problems and you enable the Allow Cross-Forest User Policy and Roaming User Profiles policy policy at Computer Configuration \ Administrative Templates \ System \ Group Policy, because of an error in Userinit.exe.

You can safely ignore this erroneous entry in the Userenv.log file.

NOTE: See How can I troubleshoot Group Policy object processing failures that occur across multiple forests?