I have a compaq presario sr1303wm desktop PC.the trouble I am having is when I turn my PC on it turns right off,then I have to unplug it to get the green light on the back to come on HELP

Sounds like a power supply, check the seating of cables from the power supply to the mother board and other items. Then check whit the manufacture and see if there is a firmware update available.

Most modern PC's have a safety that detects if the CPU is getting too hot and turns the PC off to keep it from frying. Take a look inside your case at the CPU's cooling fan and heat sink fins. If they're really dirty and blocked with pet hair or dust you need to blow it out with canned air to get your cooling back or if the fan is slow or not moving at all it may need to be replaced or unblocked (I have seen stuff get in the fan blades and stop them.