Preheat oven to 180 degrees C. Grease bottom only of a 20x30cm baking dish. Sprinkle the marshmallows evenly over bottom of pan. Thoroughly combine completely thawed strawberries with jelly crystals; set aside.
In large mixing bowl, mix flour, sugar, baking powder and salt. Make a well in the centre and add margarine, milk, vanilla and eggs. Blend at low speed until moistened, then beat 3 minutes at medium speed, scraping sides of bowl occasionally. Pour batter evenly over marshmallows in pan. Spoon strawberry mixture evenly over batter.
Bake at 180 degrees C for 45 to 50 minutes until golden brown and toothpick inserted in centre of cake comes out clean.