Computer shuts down automatically

This has me stumped, I have tried everything that I can think of only to no avail. Just when I think I have it working properly, BAM, It reboots without any warning and sometimes reboots just after logging in. Can anyone help? I have service pack 1 installed. Could this be the problem?

This has gone on for over 2 weeks now and my wife (its her computer that is doing this) is getting irrate with it.

 

To start, right-click My Computer, Properties, Advanced tab.
Click the Settings under Startup and Recovery.
Uncheck the box that says "Automatically Restart ".

This will at least give you a blue screen with error information on it that is a starting point, anyway.

Also, go to Control Panel, Admin Tools, and llok in the Event Viewer to see what is listed in the 3 categories in the left pane. Errors are marked. See what you can find.

 

Thanks a bunch Abraxas. At least this will get me closer to finding what the problem might be.