Thanks. How do I find and access (and understand?) the windows event logs?
Choose Event Viewer on the Administrative Tools menu (this is from Windows XP). If you can't find it on a menu, start up a command line and type Eventvwr.msc. Or do Windows-R and enter Eventvwr.msc.
Look for suspicious events around the time of shutdown. Google what you find.
Here’s a place to start: http://windows.microsoft.com/en-us/windows/open-event-viewer#1TC=windows-7
The logs are easy scroll through - there are several, you should be interested in the Windows one and the Application and Service Logs. Look through them - you’ll be able to see where the machine was booted, then go backwards to see what happened before the boot. If you find an error you can google it.
If you find nothing in the logs that coincides with shutdown, then I’d start thinking it’s a hardware issue.