I thought POSITIONn semantics weren't passed to the pixel shader (only used for the rasterizer). I'm surprised it worked on the first computer.

POSITION0 isn't passed to the pixel shader, POSITION1 however is, though it still seems to being used by the rasterizer too. This would somehow explain why no pixels where drawn on my notebook. Still I'd be curious what was really going on, and why it worked on my tower. Well, I quess I'll never find out though...