Regedit.exe's functionality has been merged with regedt32.exe's.
In Windows XP and Windows Server 2003, Regedt32.exe is a small program that just runs Regedit.exe.
Regedit.exe is the registration editor for 16-bit Windows. It is used to modify the Windows registration database.
Microsoft recommends that you use Regedit.exe only for its search capabilities on a Windows NT 4.0-based or Windows 2000-based computer.
Regedt32.exe is the configuration editor for Windows NT. It is used to modify the Windows NT configuration database, or the Windows NT registry. This editor allows you to view or modify the Windows NT registry.
The editor provides views of windows that represent sections of the registry, named hives. You must use extreme caution when you use it to change registry values.
Missing or incorrect values in the registry can make the Windows NT installation unusable.