I just purchased a brand new W500... I feel like an idiot to ask this question, but I have looked at all Control Panel functions, including power settings, and can't figure it out. Whether operating on battery or 120VAC, my computer locks out to the initial log in screen after about 20 seconds of non-use. If I stop typing for just 20 seconds, I have to log in again. Very aggravating. How do I change this setting to longer interval?