If you are insistent on using DirectDraw, be aware that BltFast does not respect the clipper, and any attempts to BltFast outside the screen area will cause a crash.

If you are using BltFast, you need to implement your own clipper.

It would be a lot easier to dump DirectDraw and do your 2D game with Direct3D using textured quads or the D3DX Sprite class. Once you have your head round it, it really is far easier to use.