2000 years ago, Engineers are tinkering on LV-223 with bioweapons.
Meanwhile, in space, a Space Jockey, flying with a cargo of eggs, gets facehugged, crashes on LV-426.
Back on LV-223, there's a bioweapon outbreak.
2000 years later.
Space jockey ship on LV-426 is still crashed.
Prometheus lands on LV-223.
Nostromo picks up signal from LV-426.
Prometheus isn't a prequel in that sense. It's its own story in the same universe.