I used to play that in all of my computer classes at school, good game. For people who are wondering, this is the games' storyline: You have to protect your house from zombies, so you plant plants and they shoot shit at the zombies.
You basically Collect sun when it appears in the sky. Each sun you collect you get 25 'Sun' and a plant may cost you 25 or 50 or 100 sun to plant it. Plants shoot little pea things at the zombies and each plant has a different effect.