I presume you have a Windows OS. Maybe XP or Vista. Anyway this can sometimes happen if you have flaky power coming into your home such as power spikes. A UPS (uniterruptible power supply) to handle the voltage spikes. It could also mean that you have a power supply unit (PSU) that is failing. Open your computer also and make sure that it is free of clutter or dust that may be overheating the processor or the RAM slots. I would also take out your RAM modules and maybe swith them or just reseat them. Make sure you turn off your computer and unplug it before opening it and always wear an ati-static wrist strap so that you don't damage your motherboard or RAM modules. Overheating in a system sometimes causes the system to restart so make sure it's clean inside. If it still won't worl you may have a virus. download AVAST 4 or Antivir home edition (free versions) or go online and use some of the free online scanners. To do this, maybe when your computer is booting up hold down the F8 key and go into safemode with networking. Uninstall questionable sotware that may be starting up with your computer or disable it in startup. Go to Start, Run, type in MSCONFIG, go to the start up tab and uncheck unecessary items that start up with the Operating System.