Aligned sprites

I would like to know how I can get a polygon quad like a sprite aligned around a vector? This is for energy beams (from spaceships). I have a start point for the beam, and an end point, and I need to draw a screen-aligned sprite around it. DX docs had a sample on billboarding but it was only alinged around the y-axis, not any vector. Secondly, missile trails. I need segmented beams for missile trails such as in half-life or freespace 2. So this is basically like the above beam except there is a polyline (line strip). How do I place the vertices so that each segment is screen-aligned around its segment line, and the vertices coincide with the vertices from the previous segment?