Screen goes funny afta 15mins of playn a game

Hello,
Im no computer expert, but i have just brought the sims3, after about 10-15minutes of playing the screen goes all funny and a bar comes up and says "display driver has stopped working normally" i then have to restart the computer. sometimes i have to turn it off by the powerpoint. i have deleted heaps of stuff on the computer and empty the recycle bin, but this does not seem to help. can anyone out there help me out. and explain in simple form, as i am no computer wizz.
Thank you. JayBee xx