Have you tried restarting your computer? Also, are you positive that you are using the OpenCL poclbm miner + the CUDA (important that you use CUDA not the usual nVidia drivers) driver?
Restarting? yes. Lol, I give it a restart every time I stop mining. I just realized I wasnt very specific about the crashes. When it crashes, I mean the whole machine is unresponsive. Not even the cursor will move. Everything is completely locked up until after a hard reset. And I am pretty sure about using OpenCL. I can try to double check though. I am using the defaults if that makes a difference. And I do have CUDA installed. I mean, everything works; my hashrate is normal for the video card I am using UNTIL I try to stop it.