Depends on the cause. Is Task Manager showing 100% CPU usage? (right click on clock, select Task Manager, then more details at bottom left of task manager window) this will show if a particular app or process is causing it.
Physically, there is a very thin layer of thermal paste between the CPU chip and its heatsink, if this hasn't been applied correctly or has dried out over time then the heat can't transfer out and the CPU probably will overheat, the shutdown is to protect it.
Cure is to dismantle it and clean the old paste off, then apply new. Not a difficult job if you know how but if you don't then get your local computer shop to do it, shouldn't cost the earth (it is mostly labour costs as taking a laptop to bits is a PITA).