Segmentation Fault blitting a surface

Hi. I’ve got a surface located in video memory. This surface has alpha=128.
I lock it
to do a getPixel(). Then I unlock it and, when I try to blit it, it raises a
If I don’t lock/getPixel/unlock it, it works fine. If I place the surface in
memory it doesn’t fail. If I run the engine in windowed mode it works ok.

I’m working with WinXP, VC6.0, SDL 1.2.4, fullscreen, S3 Savage4 32Mb.

? Any ideas ?