a ) for over heating of a laptop::: simple home remedy i am using it since 5 years.....use any stool, small bath-platic chair( upside down) and keep laptop on top of it. Laptop wont restart bcoz of heating.
------------ expecting ur system not infected with anything then -------
b) for random shutdowns, one of many reasons could be multiple browsers,multiple anti virus-anti spyware,mutiple firewalls......or latest updation of os path or security or browsers of any other application that could be conflicting.
remedy to this.......if recently installed something ,uninstall it and try. if u keep restore points then u can try system restore. First check restore points, If done incorrectly to old restore points u would losse all the installations after that point.
most of the time in windows such problems arise couse of multiple browser conflict(uninstall the latest one).....use the tool WhoCrashed v 5.50 mentioned by friend above.....i most of the time faced issues with crome(for firefox use latest patch update).
if the issue is bcoz of performance of the system, kill unwanted applications running and customize ur system for best performance instead of best appearance. Your system wont restart or shutdown , on top of it, it will work better.