my pc totally shuts down with no warning or nothing?

my pc shuts down totally with no warning no nothing. I can turn it back on straight away and everything will load but other times things will load and then shut down again ( the pc i mean ), ive done the cleaning inside of the pc and the fan and all but still keep happening. There is no time limit to each shut down. My pc is a intel(R) pentium(R) 4 CPU 2.93GHz