After renaming (because you didnt wat to lose the profile) or deleteing a user profile from the "users" directory, then on trying to log on as that user you get the following error message....

You will also see Event ID 1511 logged to the machines application log.

1. Log on to the machine with administrative rights.
2. Click Start > In the Start Search box type Regedit {enter}.
3. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
4. Here you will see the SID (Security Identifier) for all the users on the machine, you will notice one has .bak on the end of it.

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5. Delete the entire key that ends in .bak then reboot and log on as the user in question.

Note To properly backup and remove a user profile in vista, take a copy of the c:\user\{username} folder, then in Control Panel {Classic View} > System > Advanced System Settings > User Profiles section > Settings > Select the profile > Delete.