"seguro de que..." is followed by a sentence: Estoy seguro de que los bancos cierran hoy. However many people leave off the "de" before "que" (this is called queísmo: http://es.wikipedia.org/wiki/Que%C3%ADsmo). "seguro de..." is followed by a noun and means "assured, certain": Estoy muy seguro de mis respuestas, I'm very certain of my answers. Ella no está muy segura de si mismo. She's not very sure of herself. Also "seguro que" is used without "estar" and means something like "probably" - Seguro que llueve hoy. It's probably going to rain today. It'll rain today for sure. ¡Seguro que no tienen la llave! - I bet they don't have the key!