Hi Honey,
It can be caused by a number of things. Does it happen when the PC has been on for a while (ie when it's warmed up), or does it do it when it's cold as well?
The USUAL suspects are faulty RAM or faulty power supply unit ("PSU"), but there can be other causes as well. Unless you enjoy stress for a living then it's probably best to take it to a computer repair shop and let them run diagnostics on it.
How old is it?