auto turn off (crashed?)

I always experience that blender auto turn off when I am working with a project. I did not experience and lag and it just stop for a moment and turned off. I am not sure whether it is a crash a not because everything run smoothly before it auto turn off. The spec of my pc is amd sempron 1.6 single core, 512 mb ddr1 ram and built in graphic card with 64 mb( The brand is something via s3g).