Click Start, Run and enter REGEDIT Go to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
Look in the right pane. Look in the right pane for the value called UserInit. It should read
C:\WINDOWS\system32\userinit.exe,
Including the comma at the end. If not, double click the UserInit value and change it. If your XP install is not on C:\ or not in the Windows folder, adjust the line accordingly.