Graphics gone crazy?

Hello, Ever since I updated; my mac has gotten a bit crazy.

I suppose it's something with the graphics, I'll post a picture because the only way I know how to describe this is "JPEG-pixel-party around all windows".

I'm not an apple pro, If I were to "reset" my macbook, do I need the operative system on a disc or any code? I have nothing, I got this computer from my highschool when we graduated since they wanted to buy a newer version for the new students. The computer was pre-installed.. It's 7 years old and have been working fine(just a little slower then before).

Btw, it didn't start as soon as I updated and I can still use it "normally".