striped out regions on the screen

Hi, Ive been using blender for about 3 years now, and up until now ive never had a serious problem with it, but now, when ever i use it, it tends to have stripes missing on objects in my files, it happens to all of them,

I have a ATI radeon xpress 200m card, i know its not great, but it runs TF2, Portal, Guild Wars, GTA SA, etc, no problem,

Help please!