1) Watch the episode of Beast Wars called Possession.

2) Watch the part where Starscream tells everyone about his past, about how Unicron destroyed him and his spark survived.

3) Later in the episode, Black Arachnia says that Galvatron killed Starscream.

4) If you watch Transformers, you'll see that Black Arachnia was right, that Galvatron did destroy Starscream, but if you watch another episode, Unicron gives Starscream his body back, so Starscream's spark can't exist because he has his own body!