In the beginning of the game, after the king is murdered, I go on my way to the sewers and I begin receiving random quests. It starts with "Horse Armor" popping up, stopping gameplay, and then another, and then another, and so on and so forth, stopping gameplay about every 20-40 seconds. I can't find anyone else with this problem! It happens no matter how often I reload the game, and I have tried uninstalling and reinstalling.

That's normal. That's because the DLC doesn't activate until after you leave the sewers.

All the DLC quests start as soon as you leave the tutorial dungeon.
This happens to everyone, and is perfectly normal.

edit: ninjad

This can help: http://www.nexusmods.com/oblivion/mods/11474/?

but it also happens in my old save game with over 80 hours of gamleplay on it. I've been playing this game since it was released and this has never happened before.