Here is an easy way to fix the problem of Lenovo Thinkpad 400 power button which doesn’t do anything while running in Vista. But first you need to know how the symptom is. Since have the new T400 someone has power button set to “turn off computer” in Power Manager setting but when hit the power button in Vista, there is nothing happened. He is using Vista 32bit SP2. The power button turns the computer on just fine and surely holding it performs a hard shut down but it does not perform any action when just pressing it.
To make the power button on your ThinkPad T400 works, Go to your ThinkVantage power manager (or you can use basic one in Control Panel) and then change the action when the power button is pressed.
Another simple solution you can try to do is holding the power button in around 2 seconds. It will cause the T400 to shut down normally (assuming that you have it set to shutdown) and holding it for around 5 seconds or more will cause a ‘hard’ shutdown that just turns off everything without letting it load.