At conversion you had the baptism of the Spirit, but that wasn't an entirely personal thing. It wasn't just for your benefit.
It is clear that every Christian has experienced the baptism of the Spirit, for Paul speaks of this as something that is in the past for every believer. This verse is a proof text to show that the baptism of the spirit occurs just once at the beginning of the Christian life and is part of our conversion experience. It is not something that happens subsequent to conversion so that there are some Christians who have had the baptism of the Spirit and others who have not – two classes of believer, as it were. It is a conscious experience for the believer, when we are united with Christ and willingly submit to a new life of obedience to him, acknowledging him as Lord.