Laptop won't turn on--flashes blue and black

So a few months ago I was typing up an essay on Microsoft Word and my screen went black. This wasn't unusual, since it happened a lot, but when I tried to turn the computer back on the monitor started flashing between blue and black. The computer got really hot, so I turned it back off and decided to let it cool down. The problem continued over the next few days, and since I didn't have enough money to get it looked at I agreed to share my roommate's computer.

Now, two months later, the computer continues to have the same problem. It turns on, flashes bright colors, and gets really hot. Then all of these weird white and grey lines pop up on the screen.

How do I fix this?

Additional details: The computer is an HP Compaq Pavilion g4 and it is four years old.