Quake 4 freezes

I am playing Quake 4 at med settings at 800x600 with shadows turned off. The gameplay is very smooth. But at certain points the game just freezes. The sound plays in the background but the picture just freezes. Sometimes it comes out of this. But sometimes a blue screen comes "Windows has been shut down to prevent further damage to ur computer." The file involved is nv4_sp.dll. I know its a file in the nvidia drivers. My rig is:
3.0 p4 ht
512MB DDR RAM
FX 5200 oc @ 290/419

I know some of u might say that my ram is not sufficient or my gpu is crappy. But even at the lowest settings at 640x480. Sometimes this happens. Otherwise the game moves at 40+ FPS all the time.