In the Event Viewer you are looking at logs for "System" and for good measure, "Application".
When this system suddenly shuts down again, make a note of the time. You already know the date. Restart the system and once Windows has loaded and is stable head directly over to the Event Viewer, (Control Panel > Administrative Tools > Event Viewer).
In the left hand pane expand "Windows Logs" and you will see a list of logs. You will want to check the System logs first, then Application logs.
Highlight System and in the middle pane the log will display. This is a listing of everything that happens under the hood.
Middle pane, top area is where the list is. You are looking for events that occurred at the time of the sudden shutdown. Highlight each to see a General description in the area below.
You are looking for clues what, if anything, caused the system to suddenly shutdown.
If there are no errors or warnings in System, check the Application logs.
I see you mention Vista Home Premium and Windows 7. Can you explain that? Which system is this problem present? Is this a dual boot computer or different computers? If dual boot, does this only happen on one OS, (which one), but not the other?
Mark