When you are loaded up from WinXP boot CD, you can edit registry on existing installation on the HDD.
- Start Regedit.
 - Go to HKEY_USERS
 - Select File -> Load Hive. Point to C:\Windows\System32\Config. Inside this folder select one of the files i.e software, SAM, system etc.
 - Type new name for the folder to mount the remote registry… like REMOTE_System or REMOTE_Software.
 - Now remote registry is loaded into regedit and you can change things there just like in normal registry.
 - After done changes, you MUST unload the hives: File -> Unload Hive.
 
Check HKEY_USERS\Remote_Software\Microsoft\Windows NT\CurrentVersion\Winlogon
Shell entry must be pointing to Explorer.exe.
Userint should point to C:\Windows\System32\Userinit.exe