For quite some time now the nvidia drivers freeze on some cards when using the RENDER extension and scrolling quickly in apps that use RENDER through Xft to render fonts.
Actually only X freezes, but on killing X, the whole machine does.
NoRenderAccel is a workaround, or lowering the clock speed with nvclock.
Better would be to decrease the clock speed a bit for the GF2 MX400 (at least) in the drivers.
I think this was reported dozens of times, why isn't it fixed yet?