sony vaio unexpected windows shut down

Hi, I bought a sony vaio VGN-N365E in July. It came with vista. For some reason, windows will shut down unexpectedly. It has happened at least 5 times. I only use the computer for email and word documents. I have mcafee and avast, ad-ware SE professional, as well as windows defender on my computer. Any ideas why it shuts down unexpectedly? Sincerely, Erica

You say you have both McAfee and Avast…two antivirus programs running at the same time is not a good idea. They will fight for your system's resources and possibly fight each other's virus definitions. Please use only one Antivirus.

Have you looked at the 'event viewer' to see what happened when your computer shut down?
You can do that by clicking on 'start' ….go to accessories, click on 'run', then type eventvwr, click OK.

You may be able to find which application is causing the shutdown, or you may need to update a driver.