Click Start, Run and enter CONTROL USERPASSWORDS2. Go to the Advanced tab.
If the option there is greyed out, go to Start, Run and enter GPEDIT.MSC. In GPEDIT go to Computer Configuration, Windows Settings, Security Settings, Local Policies, Security Options. In the right pane, locate Interactive logon: Do not require CTRL-ALT-DEL. Double click this and set it to Disabled. Its default is Not Defined.