Put glReadBuffer(GL_FRONT) once before the glReadPixels and you read what you see.
In double buffered contexts the glDrawBuffer and the glReadBuffer are both defaulting to GL_BACK.
Some tips:
After a SwapBuffers don’t assume the backbuffer contents to be still valid. There is a hint in the dwFlags of the PIXELFORMATDESCRIPTOR which says if the format copies or can swap, those are named PFD_SWAP_COPY and PFD_SWAP_EXCHANGE.
Don’t assume pixels overlapped by other windows or moved off the desktop to give useful data on readback. Those fail the pixelownership test and are mostly bogus.