If you need to delete a user profile in Vista to clear some issues, there is an added step.
1. Delete or rename the user profile in C:\Users. If you were logged in to this user, you may need to reboot your PC and log in using another administrator level account.
2. In the registry, go to HKLM\Software\Microsoft\Windows NT\Current Version\Pofiles List. Expand the key and search for the username. Delete the GUID key, which will normally contain a .BAK at the end.
If the second step is not done, you will end up logging on to the system with a temporary profile.