Personally I would approach the modelling of this truck as if I were in a sheet-metal shop:

1. Use various tools/operations to deform 2-d planes with high polygon counts
2. Add thickness to the planes, then align and group them in the assembly room.
   Then the windows would be transparent planes with a little added thickness.

In your case you can try booleans if you dare, but they seem to go wrong alot, especially if your polygon count is low. Another possibility is to use a transparency map to simulate windows.

Alternatively, if you're still in the vertex modeller, you can start deleting the approximate shape of a window with the marquee tool, then zoom in and fine-tune it by deleting individual links and vertices.