If for some reason you’d like to reset all your Windows 7 or Vista security settings to their default values you may do so as follows:
Open an elevated command prompt and type the following command:
secedit /configure /cfg %windir%\inf\defltbase.inf /db defltbase.sdb /verbose
After you run this, the standard user accounts may no longer appear on the logon screen when you re-start your computer or try to switch users.
This occurs because standard user accounts are removed from the Users group when you reset Windows security settings.
To add the affected users accounts back to the Users group, follow these steps.
Open an elevated command prompt.
In the Command Prompt window, type “net users” and hit Enter. A list of user accounts will be displayed.
For each accountname listed in the Command Prompt that is missing from the log on or switch user screen, type the following command and hit Enter.
net localgroup users accountname /add
In Windows 7 & Vista, the Defltbase.inf file is a Security configuration template for the default security. You can view the settings for this file in the following location: %windir%\inf\defltbase.inf
Alternatively, you can simply use this Microsoft Fix it 50198 to let do all this, automatically.