Click on Start > Click Run > Type "control userpasswords" (without the
quotes) > Click on OK
This should open the "User Accounts" Applet. If not see if you can open the
alternative.
Click on Start > Click Run > Type "control userpasswords2" (without the
quotes) > Click on OK
If you can open the first one then I would run the system file checker.
Click on Start > Run > sfc /scannow > Click OK > You may need your XP CD >
This will check all major system files and replace any that are missing or
corrupt.