I've decided to reinstall Oblivion, get rid of all the mods and start over. So I do that, I uninstall Oblivion, make sure all the old files are deleted, then I reinstall it and then reinstall the Shivering Isles disk. So far so good. I go to get the latest patch, 1.2.0416, Shivering Isles version. I start to patch, then it says 'Old File Not Found. However a file of the same name was found. No update done since file contents do not match'. Can anyone explain why this is happening?

If you have GOTY edition, it is already patched up. If not, make sure you are grabbing the correct patch for your game (SI), and your region/language.