click 'start/control panel/performance and maintenance/system/advanced/start up and recovery/settings' or 'start/control panel/system/advanced/start up and recovery/settings' and untick 'automatically restart'
now instead of a restart you will/should see a useful error displayed, note this down and post it here or look it up in the 'searchKB' box click here
also have a look in 'start/control panel/performance and maintenance/administrative tools/event viewer' or 'start/control panel/administrative tools/event viewer' and click on the 'system' and 'application' tabs and look here for any errors displayed marked with a red x, click them for more information. may help.
is windows also up to date? click here