OpenGL different rendering on different PCs

I'm trying to make something of a UnitTest library for my renderers, and I've just tried running it on a different PC. Unfortunately, that PC gives different results when rendering. - The first issue I noticed is that the same clear color gives a different pixel value on the two PCs. - The second issue I found is that triangles are rasterized differently. I have a simple triangle, with no matrices (projection, or modelview), no lighting, no texturing, and on one pc one of the edges is extended exactly 1 pixel. I can provide examples if needed, but surely this kind of thing shouldn't be possible? In this case, both cards are NVidia, although the OSs are different (Vista vs XP). Are there any settings which might influence something like this? The triangle has a vertical side, a horizontal side, and the third side is exactly 45 deg. So I don't think alias settings should effect this. Edit: I've tried it on a third pc now with an integrated Intel graphics chip, and it returns the same as the first pc.