Windows XP Logs on, then Logs off
You need to restore the file userinit.exe to C:\WINDOWS\system32\ and restore the Registry data:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon]
“Userinit”=”C:\WINDOWS\SYSTEM32\userinit.exe”
Go to regit on another networked computer - registry - connect network registry -and enter the computer name with the problem. You can edit the registry that way .
Or take the hard drive out, connect via usb, and load registry hive that way.
Or Download tool http://www.ms-mvp.org/