Do you want to know when did your computer shut down last time? You can use Event viewer in Windows Vista, Windows 7 and Windows XP.

1- On Winsows 7 and Vista Click start icon and type Event Viewer, now click on Event Viewer

    On XP press Windows key + R and type eventvwr.msc and click on OK.

2- Now click on Windows Logs, then double click on System. ( On XP simply click on System on the left pane)

After some times all recent events occured on your system will be appeared in descending order w.r.t. time.

3- Now locate the 13 (On Vista and XP it is 6006) under Event ID (13 is event id for shut down), note the corresponding time for that event. This is the time when your computer shut off last time. From the following image you can see that 2:50:19 AM on 2/13/2010 was the last time my PC was shut down.