Seathe is located inside the Duke's Archives in Anor Londo. From the bonfire near the starting of Anor Londo, go into the left building (not crossing the bridge to the stairs) and follow the path through the hallway and up the lift at the end. Once you find your way to Seath, you will not be able to fight him because you must be defeated (yes you lose everything as per normal). You will then appear in a cell. Fight your way through the archives and find the Crystal Caves. He will be located inside there.
Once the fight starts with Seath, run behind him and look for a cystal to smash. This will negate his health regeneration and allow you to kill him. After that he isn't very difficult, just take your time, but watch out for his crystal attack because it will curse you if you stay in them too long.