Im guessing that your GPU is fried. So literally, each time you run your computer, then openning a game forces more power to go through your GPU causing it to overheat due to insufficient airflow.
The insufficient airflow may be caused due to the computer being old, dust clogging up on the fan and stopping it from rotating and max speeds, if the fan spins slowly, the heat builds quicker before it can be blown out of the rig.
Firstly I would turn off your computer. Wear some plastic gloves i think it was so you dont short circuit any hardware, open your computer, find the graphic card, disconnect the graphic card from its slot, you will need a screwdriver then take a closer look at it. If the fan is clogged with dust either unscrew the fan and clean it or get some comperssed air and blow it around the fan to get rid of the dust.
Also check all the parts of your computer where air leaves the rig. Make sure its not covered by dust. If it is, use a vacuum cleaner to remove the dust.
If you need more help post here or PM me.