To restore your operating system to the original installation default security settings, follow these steps:
Click Start, click Run, type cmd, and then press ENTER.
In Windows XP, type the following command, and then press ENTER:
secedit /configure /cfg %windir%epairsecsetup.inf /db secsetup.sdb /verbose
In Windows Vista/7, type the following command, and then press ENTER:
secedit /configure /cfg %windir%infdefltbase.inf /db defltbase.sdb /verbose
You will receive a “Task is completed” message and a warning message that something could not be done. You can safely ignore this message. For more information about this message, see the %windir%SecurityLogsScesrv.log file.
DO NOT REBOOT OR LOG OFF YET
The next step after you run the above fix, is that standard user accounts may no longer appear on the log on screen when you 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:
1. Click Start, click Run, type cmd, and then press ENTER
For Windows Vista/7:
1. Click Start, click Run, type cmd, and then click Run As Administrator (Windows Vista/7).
For all OS’s:
2. In the Command Prompt window, type net users and then press ENTER. A list of user accounts is displayed.
3. For each account name listed in the Command Prompt that is missing from the log on or switch user screen, type the following command and then press ENTER:
net localgroup users account name /add (e.g. net localgroup users Administrator /add).
This should allow you to log back into the machine once you restart to apply any changes that have been made to the system with the above steps.