My Tiny computer switches off?

My Tiny computer started to switch itself off a few months ago so i changed the power supply. This did not solve the problem. I then thought i may have a virus from music downloads so installed AVG, Spysweeper, Registry repair and ETD security scanner. This did not fix the problem either.I can listen to a few songs using Media player but a full album makes the power go off and the same with a DVD. I have also hoovered any dust from circiut boards and heat sinks.The power only goes off when i run large files or use a few applications at a time from what i can gather. The memory is no where near full. Please can somebody shed some light on the situatioin. thanks