Safe Mode with Command Prompt still requires you to log on.
If you haven't turned it over to a shop yet, your best bet is the Vista RE disk; load the software hive and check that registry key and copy the userinit file from the XP CD, or one of the other locations, as outlined in the post I linked above (in post #6)
You can open
two copies of Notepad (Type
C:\Windows\Notepad), then click
File | Open on both, and use the Open File dialogs as Mini Windows Explorer windows to drag and drop files between the open dialogs.
A Vista RE CD image can be downloaded from one of these links:
Vista Recovery Environment CD 64 bit Vista 32 bit Vista