User Account Controls

I just bought a new PC with Windows Vista Home Premium in it. It had been alright during the first few weeks until this time when I can no longer access the User Accounts and Family Safety as well as the Security Menu in the Control Panel. There is a shield besides the Allow a Program through Windows Firewall in the Security Menu and also shields beside Set Parental Controls for any User and Add or Remove User Accounts in the User Accounts Menu which suggests that I am unable to change anything on it although I am the Administrator.

I tried every suggestion I've come up on the web but it seems it doesn't work.

Can anybody help me sail through?

Thank you.

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