Check your power management settings in control panel/power options
There are also viruses around which will shut down your system so this is
another possibility.
Do you have the lateste security updates from windows update
Do you scan for viruses, if not heres a free one www.grisoft.com
and a free firewall www.zonelabs.com
I use these, there are others just as good, I just happen to know the urls
to mine.