I have previously had this problem and found out on looking in the event viewer there is the following
The process Explorer.EXE has initiated the restart of MyComputer for the following reason: Other (Unplanned)
Minor Reason: 0x0
Shutdown Type: shutdown
Comment:
For more information, see Help and Support Center at
http://go.microsoft.com/fwlink/events.asp.
I have up to date virus scan software and have tried this and since then have not experienced any shutdowns since
you can try this see if it helps at your own risk
i disabled hibernate by doing the following
Click Start, settings, control panel, Power Options,
this opens up a new window called Power options properties, left click hibernate and uncheck the Enable hibernate check box and click apply and then ok
since this has helped me it could help you too
but worth a try
