switching users with XP Pro

Sometimes when users try to switch their accounts or someone tries to log on while the system is on, they can’t log on because it keep saying that your password is wrong and this problem applies all the accounts(5 account) except the admin.account.The only way I can solve this problem is if I shut off the power while the system is on, not the normal shut down and then restart the system so that the scandisk will be activity through the restart.
If the restart skips the scandisk then it becomes like a normal startup and didn’t solve the problem, after the restart through scandisk everything works fine.
I wonder why the admin.works but not others?