My screen frequently glitches and my computer crashes.

I've had it since August, and it's always been a little bit glitchy, but in the past few weeks it's become unusable. Doing anything on it will cause the screen to start flickering on and off and shifting colors, and cause patterns of green dots or squares to cover the screen. After a few seconds of this, it shuts down. Often times, the lights on the tower will light up, but my keyboard, mouse, and monitor stay off. I'll include some images

, and I can provide more details if needed. Thanks in advance!