You may not think it's overheating, but I would still recommend that you open your computer case and make sure all the fans are spinning okay and that there isn't an excessive amount of dust in there that would impede air flow. See the following Bleeping Computer
tutorial for guidance on cleaning inside your computer.Cleaning the Interior of your PC
Also, have a look in the Event Viewer
for any errors at the time of the shut-downs.
To open the Event Viewer go to Start
> Control Panel
> Administrative Tools
> Event Viewer
. Alternately, go to Start
and type in "eventvwr.msc" (without the quotes) and press Enter.
Check in all the categories (Application, Security, System and Internet Explorer).
If you find an error that occurred at the time right-click on it and select properties. Copy the information in the window and post it back here. This will help us diagnose your problem.How To Use the Event Viewer