ArcGIS 9.3 vs 9.3.1 Service Packs

Hello all. I cannot remember if 9.3.1 is just 9.3 with service pack 1 or was 9.3.1 it's own standalone product?

I have a copy of my 9.3 software. I did a google search on service packs and for this specific version, I see they only had SP1. There is a service pack 2 for 9.3.1. So I am think I install 9.3 software and then I should update 9.3 SP1 which I'm think is the core version of 9.3.1 and then I update 9.3.1 SP2? Can someone confirm this or correct me if I'm am wrong?

Thanks,
Mike