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You could try a manual install with files supplied by another user who has Yesterhaven installed.

After installation the Yesterhaven files are located in a folder where Dungeon Siege is located. There's Yesterhaven.dsmap, Yesterhaven.dsres and Yesterhaven_ReadMe.rft. A custom shortcut launches the add-on.

If no one else can help you, I can supply these files as I have it installed in my copy of Dungeon Siege. As it is a "free" addon to Dungeon Siege, there's no legalities associated with distributing it this way.