A bit of a gotcha with using the Active Directory Users and Computers tool on Vista is that it (very sensibly) protects the objects you create from accidental deletion but (very annoyingly) doesn’t inform you that it’s done this. If you are using the AD Tools on Vista and you suddenly find that you can’t delete something you created, then check the Object tab on the object in question. If the “Protect Object from Accidental Deletion” box is checked as shown here within the red circle:

… you’ll need to uncheck it before you can delete or move it.